Christopher Tollefsen writes arguing the intrinsic evil of the death penalty, here.  He forwards reasons that he deems sufficient to relegate the consensus of the fathers and doctors, and prior ecclesial judgments regarding the death penalty, to the dustbin of history along with prior acceptance of slavery, and buttresses his argument with an analysis of intention (in support of the proposition that God cannot intend death),   Here  I will argue that his analysis of the tradition (e.g., the oath required of the Waldensians to re-establish ecclesial communion), and the likenesses he draws of the death penalty with slavery in terms of the tradition simply cannot be sustained.  To be unequivocal, I will put it straightforwardly:  any claim that the death penalty is absolutely and intrinsically unjust contradicts the consensus of the Fathers and Doctors and numerous prior papal teachings. The likeness drawn with support for slavery is unsustainable. The teaching that the death penalty is intrinsically evil seems very directly to contradict Scripture and Tradition.  The claim that the death penalty is intrinsically evil also poses profound challenges to--perhaps even contradiction of--the Church's teaching regarding the nature of the redemption; the nature of penalty as such including the penalty of final damnation; and the Church's traditional understanding of human dignity itself as defined by the teleological ordering of the imago dei of representation to the imago dei of conformity with God. 

With respect to the tradition, Tollefsen argues as follows:

Moreover, and this is a key point of Long and others, the medieval followers of the heretic Peter Waldo were made to swear an oath by the Church, asserting: “Concerning secular power we declare that without mortal sin it is possible to exercise a judgment of blood as long as one proceeds to bring punishment not in hatred but in judgment, not incautiously but advisedly.” Long finds a “high theological note” in this oath, and asserts that it indicates that “as such it [the death penalty] cannot be malum in se”, i.e., intrinsically wrong.

But there is a considerable difference between swearing that a type of act is not intrinsically wrong, and swearing that those who undertake that kind of act, under particular circumstances, and given certain states of mind, need not commit mortal sins in doing so. Put another way, the oath only requires an affirmation about the subjective state of those who pass or execute lethal sentence, and this subjective state is one we should expect to find in a culture that uniformly accepts the death penalty as just and indeed divinely permitted.

But the oath imposed on the Waldensians went to the intrinsic moral permissibility of a judgment of blood.  To say it was only about a judgment of blood "under particular circumstances" is misleading. The very question at stake was whether under any circumstances so grave an act could be moral. The Church's requirement that the Waldensians swear that under some circumstances, the penalty may be imposed without mortal sin, was a response to the Waldensian judgment that imposing the death penalty is intrinsically evil, a malum in se, such that deliberately and voluntarily imposing it is always evil.  It does not address merely the subjective state of those imposing it.  On this score, his description of the actual oath required of the Waldensians seems to be fatally inaccurate.  The oath was formulated precisely to exclude the position that Tollefsen apparently holds.  Cf. Enchiridion Symbolorum (Denzinger, 30th edition, tr. Deferrari; New Hampshire:  Loretto Publications, 1954) #425, p. 168:

De potestate saeculari asseriumus, quod sine peccato mortali postest iudicium sanuinis exercere, dummodo ad inferandam vindictam non odio, sed iudicio, non incaute, sed consulte procedat.

Concerning secular power we declare that without mortal sin it is possible to exercise a judgment of blood as long as one proceeds to bring punishment not in hatred but in judgment, not incautiously but advisedly.

Someone might wish to argue to the contrary:  e.g., that these words are compatible with addressing the case of someone imposing the penalty who is invincibly ignorant.  But that is very clearly not the nature of the imposed oath nor the purpose for which it was imposed.  Directly to the point:  how does one "advisedly" and "not incautiously" perform a malum in se?  Can one even imagine the Church saying about, say, adultery, that "without mortal sin it is possible to exercise the passions of the flesh with one's neighbor's wife so long as one proceeds to the act of adultery not purely in lust, but in judgment, not incautiously but advisedly"?  Would Tollefsen swear such an oath, on the supposition that it merely concerned one's "subjective state"?  Or suppose this to be an intelligible formulation?  No more is it an intelligible rendering of the oath required of the Waldensians by the Church, which manifests a high theological note that must be observed when considering the fonts of this question in the tradition.  It is utterly frivolous to fail to see that this formulation asserts that the death penalty is not an intrinsic evil.  Why does it manifest a "high theological note"? Because the Church required this as a condition for the re-establishment of ecclesial communion. That is to say, for so long as the Waldensians held that the death penalty were intrinsically evil, they could not be in union with the Roman Catholic Church.  And this is because the view that the death penalty is intrinsically evil was considered to be gravely at variance with Catholic teaching. This is far from a mere "permission"--this is an ecclesial judgment that the claim that the penalty is essentially immoral is not compatible with Catholic faith.  Regarding any view that the death penalty is intrinsically and absolutely evil (as distinct from general prudential arguments against its application), to the present day there remain very strong reasons to hold that such a view is incompatible with Scripture and Tradition, and with the universal teaching of the Church articulated by the Fathers and by many pontificates (not excluding either Pope John Paul II, who never included it in the list of mala in se in Evangelium Vitae, or Pope Benedict XVI, who when he was prefect of the CDF famously wrote Cardinal McCarrick that dissent over the prudential aspect of John Paul II's teaching on the death penalty was licit and could not be compared to rejection of the Church's teaching on abortion or euthanasia).

Tollefsen also argues:

This is an important point for the larger Catholic question of capital punishment and the development of doctrine. Some worry that if the Church can develop on this, it can develop on anything. But there is a crucial difference between Church teaching on capital punishment and Church teaching on, say, abortion. The Church has always—always—taught that abortion is never to be performed, or sought after. Failure to abide by this teaching is considered a mortal sin, and incompatible with life in God’s love. But the Church has never taught that the death penalty is mandatory, nor has it taught that one sins if one rejects its permissibility (and as we’ve seen, this was not what the Waldensians were required to recant). At most, the Church has taught, and perhaps only accepted, its permissibility.

Note well how the argument has changed!  He starts by insisting that the death penalty is a malum in se, is intrinsically evil.  But if the penalty is intrinsically evil then it is precisely to be compared with abortion, something the Church has expressly rejected by formally and constantly affirming its moral validity.  Further, the Church has never said that abortion may without sin be pursued provided it is sought "not in hatred but in judgment, not incautiously but advisedly".  By Tollefsen's logic, since someone invincibly ignorant might not be culpable of the full malice of the act of abortion, the Church could require for the re-establishment of ecclesial communion an oath indicating that abortion is possible without mortal sin so long as it is performed "advisedly" and "not incautiously".  This is an impossibly contorted construction and simply untrue to the tradition.  

To the contrary, for the Church to teach the permissibility of the death penalty is for it to deny that it is a malum in se.  Further, this denial has been doctrinally clear and conspicuous.  It proceeds through many pontificates, it is shared by all the fathers of the Church, and it is found in indefinitely many approved catechisms, in addition to seemingly being present in Scripture itself (at least in the way that the Fathers read Scripture, which we are told by Trent--in its section on the use of the sacred books--is the way that we should read it).  Pius XII taught that the penalty was morally valid and not subject to cultural variation (Acta Apostolicae Sedis 47 (1955): 81-82.)

Tollefsen argues:

But so, seemingly, did the Church teach, or at least accept, the permissibility of holding slaves or coercing heretics. With regard to these practices, the Church’s teaching has, in developing, grown stricter, not more lenient. Even her teaching on religious liberty is not a freeing of what was once bound, but rather involves a new clarity on the limits that responsible political authority must accept. Moreover, those developments have proceeded by much the same path that the development, if such it is, on the death penalty has: by deepening insight into the nature and dignity of the human person, and the inviolability of human life. So it seems to me that a Catholic can accept the idea of development here without anxiety about other “mutations” of Church teachings.

Here is, from the vantage of the tradition, a compendium of error.  There is no comparable requirement imposed by the Church upon persons seeking to restore ecclesial communion that they formally swear an oath accepting the moral permissibility of slavery.  There are not multiple pontificates formally teaching that the practice of slavery is essentially just.  There are not innumerable catechisms teaching this.  Nor do all the Fathers of the Church teach it.  As for coercing heretics it is unclear what Tollefsen wishes to say, and so addressing this would run the risk of being even more unclear.

As for the claim that the opposition to slavery develops as does the opposition to the death penalty, as predicated upon "deepening insight into the nature and dignity of the human person, and the inviolability of human life"--sed contra.  Much of the contemporary opposition to the death penalty marks a regression of the understanding of human dignity so severe as to be virtually incompatible with the teaching of the Church regarding the imago dei in man. For according to tradition--to the Fathers and Doctors--the inceptive or initial imago dei of representation (the possession of rational nature, entailing in man spiritual faculties of intellect and will) is ordained to the imago dei of conformity with God, and its dignity principally consists in this ordering to noble good, common good, both in the order of nature and in the order of grace.  The initial dignity of man exists for the sake of the acquired dignity of man, and culpable failure with respect to this last merits penalty which is required not only for the common good, but to redress the wrongdoing of the sinner.  Acquired dignity is more central to man than the initial dignity of rational nature, which last is a dignity at all only and precisely because of its ordering to natural good and susceptibility to divine elevation and grace.  The initial dignity is a dignity because it is ordained to the acquired dignity which is more perfective than the first.  Yet much opposition to the death penalty proceeds by absolutizing this initial dignity rather than realizing that its perfection is found in conforming to God, and so failing to perceive that deliberate grave evil is contrary to this conformity and merits penalty.

Thus it is only because of the possession of rational dignity that the issue of penalty arises at all:  one does not penalize the earth after a particularly destructive earthquake.  Moreover, the Church's teaching regarding retribution as needing to be proportionate to the gravity of the offense is sustained throughout the tradition, up to and including Evangelium Vitae.  

One must pause to note that It is possible to think--although it is not necessarily and certainly true nor can it be always obligatory for Catholics to think--that general prudential circumstances might make the death penalty to tend in a way contrary to human dignity.  This would be a prudential judgment, and one that is as such--as prudential--intrinsically subject to variation of judgment, because prudential circumstances alter.  This is particularly true inasmuch as penalties are determinationes, determinations of prudence. Human penalty both looks back to the gravity of the offense, imposing a punishment proportionate to the crime, while looking forward to the needs of the common good, seeking to manifest in society a transcendent norm of justice, and also where and as befitting to pursue the quite different ends of conversion or rehabilitation of criminals, and of deterrence of wrongdoing. This last end of penalty is implicitly denied by many today.  But it is a matter both of the traditional understanding of the Church, and of the very nature of temporal penalty as such, that it look to the common good, one aspect of which is the deterrence of wrongdoing precisely through the application of penalty.

As St. Thomas puts it:

Respondeo dicendum quod vindicatio intantum licita est et virtuosa inquantum tendit ad cohibitionem malorum. Cohibentur autem aliqui a peccando, qui affectum virtutis non habent, per hoc quod timent amittere aliqua quae plus amant quam illa quae peccando adipiscuntur, alias timor non compesceret peccatum. Et ideo per subtractionem omnium quae homo maxime diligit, est vindicta de peccatis sumenda. Haec sunt autem quae homo maxime diligit, vitam, incolumitatem corporis, libertatem sui, et bona exteriora, puta divitias, patriam et gloriam. Et ideo, ut Augustinus refert, XXI de Civ. Dei, octo genera poenarum in legibus esse scribit Tullius, scilicet mortem, per quam tollitur vita; verbera et talionem (ut scilicet oculum pro oculo perdat), per quae amittit corporis incolumitatem; servitutem et vincula, per quae perdit libertatem; exilium, per quod perdit patriam; damnum, per quod perdit divitias; ignominiam, per quam perdit gloriam.

I respond that it should be said that vengeance (“vindicatio”—not quite what we mean today by vengeance, the virtue perfecting the inclination to remove that which is harmful) is lawful and virtuous so far as it tends toward the prevention of evil. Now some who are not influenced by motive of virtue are prevented from committing sin, through fear of losing those things which they love more than those they obtain by sinning, else fear would be no restraint to sin. Consequently vengeance for sin should be taken by depriving a man of what he loves most. Now the things which man loves most are, life, bodily safety, his own freedom, and external goods such as riches, his country and his good name. Wherefore, according to Augustine's reckoning (De Civ. Dei xxi), "Tully writes that the laws recognize eight kinds of punishment": namely, "death," whereby man is deprived of life; "stripes," "retaliation," or the loss of eye for eye, whereby man forfeits his bodily safety; slavery," and "imprisonment," whereby he is deprived of freedom; "exile" whereby he is banished from his country; "fines," whereby he is mulcted in his riches; "ignominy," whereby he loses his good name.

As determinations several penalties might in some case be essentially valid. But in certain circumstances there will be particularly important prudential reasons for favoring one over another for the sake of the common good.  And one such consideration is precisely that of deterrence.  Inasmuch as the penalty is--as is taught by universal tradition and by sacred scripture--retributively just and valid, in those circumstances in which imposition of the death penalty would most probably deter many further grave offenses, to privilege the life of the malefactor over the lives to be lost by failing to impose the just penalty does not appear particularly charitable.  Mercy presupposes justice, and justice does not exclude the death penalty.  While circumstance may prudentially preclude it--and may even generally do so, as occurs in regimes that fail in justice by treating all goods as private goods, in the reductionist culture known as "the culture of death"--it may also indicate it. The ability to restrain the felon who merits the death penalty in justice can be consistent with granting mercy; but it is not only the possibility of future acts of wrongdoing on the part of the criminal (wrongdoing which can continue even in prisons), but the likelihood of the imposed penalty to deter future such acts on the part of others similarly inclined, which must be considered.  

Deterrence is in charity one of the medicinal purposes of penalty for the sake of the common good. However, prudential opposition to the application of the penalty is always possible.  Penalty is a determination of prudence, and in some cases the imposition of the penalty may well be contrary to human dignity.  But it is not of its very nature contrary to human dignity.   The felon crucified with Our Lord who acknowledged deserving the penalty of death for his crimes was reconciled  in the very act of his own execution with God, and his acknowledgement that he deserved the penalty was not rebuked but accepted by Christ. It beggars the imagination to believe that Our Lord's utterance was an aposiopesis awaiting closure over the centuries only by the judgment of contemporaries who take themselves to have discovered that no crime can justly merit imposition of death and now implicitly would put words into Our Lord's mouth that He never spoke.  

Those who speak of the penalty as contrary to human dignity not in the context of some general prudential circumstances, but rather as absolutely and intrinsically contrary to dignity and so as a malum in se, implicitly anathematize the entire prior tradition of Catholic teaching.  They propose an account of human dignity that affirms that this dignity consists not in the ordering to transcendent common good both in the natural and supernatural orders, but rather indwells humanity in such a fashion that grave penalty for serious disorder is difficult to reconcile with it.  But this last does not sufficiently realize that the native and inceptive dignity of the human person consists in, and is specified by, its teleological ordering toward natural and supernatural good:  that the native dignity of the person is ordained to the superior acquired dignity of conforming to God and the eternal law. 

Were it true that the chief dignity of the human person is not acquired dignity but merely the possession of human nature, then everlasting physical penalty--clearly and infallibly taught by the Church--would be morally unintelligible.  But to the contrary, it is the defined teaching of the Church that those who perish in mortal sin are subject not only to eternal pain of loss, but even to everlasting bodily torment.  If human dignity absolutely and as such rules out penalty of death, how much more must it rule out everlasting penalty?  Who would not prefer death to everlasting suffering? Which is worse, the death penalty, which may be suffered in such a way as to remit sin and move one to the embrace of God Himself in time and for eternity (as scripture manifests in the instance of the criminal to whom Our Lord promised beatitude); or everlasting torment?  If we are to affirm that human dignity as such is incompatible with the death penalty, how can such dignity be compatible with, rather than immunize from, not only pain of loss but everlasting bodily torment?

Further, Christ suffered the penalty which we all have merited:  penalty of death.  But if penalty of death is wrongful in se, then God cannot justly have imposed it.  

Here we come, finally, to Tollefsen on intention.  And regarding this last, all that can be said is that he takes the object of the exterior act to exclude the integral nature and per se effects of what is chosen.  But St. Thomas Aquinas does not.  Nor, in fact, does the tradition, something that may be shown by analysis of the teaching of Aquinas and Ligouri in depth, as likewise by the analysis of the doctrinal assertions of the magisterium. E.g., as Thomas makes clear, someone stealing a precious object that is located in a Church is by that fact alone guilty both of theft and of sacrilege (cf. De malo, q. 2, art. 6, ad 2) . The thief noting only the preciousness of the object, and indifferent as to its accidental locale, may by reason of ignorance have a lesser culpability, but this does not go to the nature of the sin itself.  In any case, the integral nature and per se effects of what is chosen being included within the object of the external act pertains directly to Christ's voluntary offering of His life on the Cross.

St. Thomas uses "intentio" in different yet interrelated senses:  1) intention of the end as the ratio requiring the determination of the means; 2) intention of the end precisely through the means, and so extending to the object of the external act, which however is a secondary sense--because absolutely speaking intention of the end is necessarily prior to choice of the means and so prior to the intention of the end through the means; 3) intention as pertaining to any part of the motion toward the end.   While Tollefsen refers to intention of end and means, it seems that he does not sufficiently discriminate the nature of the chosen means:

But—like Aquinas—I hold that this free acceptance is a different act of will than the act of intending, in which an end is willed, and a means (or several means) chosen for the sake of bringing about that end. What Christ chose was perfect obedience to the Father for the sake of effecting a redemption of sinful humankind and a reconciliation of human and divine persons.

The above passage discriminates the end (effecting redemption in accord with the Father's will) but does not discriminate the chosen means precisely required by the Father's will:  the voluntary offering up of His life--a life that, as a divine person with a human nature, could not be taken from Him against His will.  Scripture is quite express on this point.  Thus John 10:18:  "No man taketh it away from me:  but I lay it down of myself, and I have power to lay it down: and I have power to take it up again." Death is not intended as simply a good in itself; yet, as annexed to a good, it may be intended as essential to the object of the external act:  because the integral nature and per se effects of action are included within the object of the external act.  Christ intended to do the Father's will, which was that He give His life as a ransom for many. The nature of the redemptive act matters.  Tollefsen continues:

But in doing so, Christ clearly willed, as accepting, the side effects of perfect obedience, in this case, suffering and death.

But it must be noted that Christ willed to accept these effects when He had the power to prevent them.  That is to say, He chose to undergo them, and indeed undergoing them is presented as essential for conformity to the Father's will. Drinking of this chalice was required of Him by the end of conforming to the Father's will.  To call that which He is commanded to do a "side-effect" is to strain not only the English but the Latin tongue. If I send my son to the store for a jar of pickles, it is not accidental to his conformity to my will that he intend to obtain a jar of pickles.  When the Eternal Father wills that Christ offer up His life as ransom for many, it is not accidental that Christ intends (in the secondary sense of "intend" that pertains to the object) to offer up His life as ransom for many.   He delivered Himself up to death, and that it was death to which He delivered Himself was not a side-effect.  Indeed, were Tollefsen's analysis correct, one might equally argue that--since the oblation is necessary to our redemption--that our redemption is a mere side-effect of the Passion (as opposed to being its essential purpose).  But it is not a mere side-effect:  the Passion and Death of Christ is that which achieves our redemption.  The Incarnation, occurring in passable flesh, from the beginning was willed for the sake of this oblation.  Christ willed to obey the Father's will for the glory of the Father, and for the redemption of mankind, and this will included, as its integral nature and per se effects, the intentional, free, and deliberate yielding of Himself to death on the Cross.  The Cross and its suffering were not intended and willed by Christ as per se will-able--as desirable simply in and of themselves--but intended and willed as essentially required for the sake of conformity with the Father's will and the redemption of mankind. 

Tollefsen continues:

Aquinas explains to us precisely why Christ’s voluntary acceptance must be invoked: Christ could have prevented his own suffering and death, and in this sense may be said to be an indirect cause of his own death by having allowed it. But the direct cause was those who brought about his death by their very intention: they chose to kill him to serve their various purposes.

It is true that Christ is not the cause of His own death save inasmuch as He wills to suffer it rather than prevent it as lay within His power. But he wills to suffer it because it is His Father's will that the redemption occur through the sacrifice.  Not everything that is not intended as an end is merely a "side-effect".  Christ did indeed choose to undergo death, to offer Himself up to death.  To exclude the offering of His own life from the external object of Christ's action, is to depict a different action.  Sed contra, Christ chose to offer Himself up to death.

In this way, Aquinas’s treatment of Christ’s Passion and death returns us to the issue of God’s intentions and the death penalty. For Christ’s relation to his Passion turns out to provide exactly the model of God’s relationship to human death I have argued for: God does not adopt human death as a means to any purpose, and thus does not intend it. But human death is accepted by God, for example, as the natural consequence of sin, which separates human persons from God’s protection.

"God does not adopt human death as a means to any purpose and thus does not intend it."  But God does adopt human death as a means of penalty--which is why we suffer it (the death we suffer is a penalty merited by the Fall); and the divine Person of the Word wills to embrace it and yield Himself up to death, precisely as the means of our salvation.  This last is de fide.  Christ does indeed "adopt" death as "means to the purpose" of our salvation, not as suicide, but as freely willing to suffer death as the means of our redemption.  "Side-effect" is inaccurate, because that which is willed as means is not merely "side-effect" and Christ willed to undergo His passion and death as means of our salvation.  That He did not cause His own death does not in the least imply that He did not freely intend and will to suffer death precisely as the means for our salvation.  

Thus it becomes clear how grave are the implications cascading from misunderstanding of the unified voice of scripture and tradition regarding the principled legitimacy of the death penalty for grave crime.  To say that the death penalty is a malum in se requires: negating the high theological note present in the Church's requirement that the Waldensians admit the principled legitimacy of a judgment of blood (for so long as imposed "advisedly" and "not incautiously"--language wholly incompatible with any thought that the application of the death penalty is always and everywhere unjust); revoking the Council of Trent with respect to the role of the Fathers with respect  to the interpretation of Scripture (e.g., Romans 13:4); contradicting numerous pontificates upholding the essential validity of the penalty; contradicting Scripture itself; and accepting a novel account of human dignity according to which the chief dignity of man is merely his inceptive dignity, the imago dei of representation, as opposed to the teaching that man's chief dignity consists in the divine ordering of man to common good both natural and supernatural whose summit is the imago dei of conformity to God in beatific vision.  For the very thing that constitutes our initial rational dignity, is precisely its ordering to noble good naturally and supernaturally; and so it is that dignity itself that requires the application of penalty when man culpably rejects and falls from the divine good.  Because the acquired dignity is ultimately more determining and perfective than our inceptive dignity--because that inceptive dignity is a dignity only because it is ordered to noble good naturally and supernaturally--culpable defect contrary to this order requires penalty.  Such penalty is not unjust:  it is due.  

One must recall, that our merely human penalties are more medicinal than retributive:  but divine penalty, howsoever much knowledge of it may and does deter man from sinning in this life, is in its application in the next life fully retributive.  Thus Summa theologiae, II-II, q. 66, art. 6, ad 2:

Ad secundum dicendum quod poenae praesentis vitae magis sunt medicinales quam retributivae, retributio enim reservatur divino iudicio, quod est secundum veritatem in peccantes. Et ideo secundum iudicium praesentis vitae non pro quolibet peccato mortali infligitur poena mortis, sed solum pro illis quae inferunt irreparabile nocumentum, vel etiam pro illis quae habent aliquam horribilem deformitatem. 

To the second it should be said that punishments of this life are more medicinal  than retributive. For retribution is reserved to the Divine judgment which is pronounced against sinners "according to the truth" . And thus, according to the judgment of the present life the death punishment is inflicted, not for every mortal sin, but only for such as inflict an irreparable harm, or again for such as contain some horrible deformity.

 It is, then, difficult to reconcile the claim that the death penalty is absolutely and universally contrary to human dignity with the Catholic faith.  For the Catholic faith affirms the justice of retributive penalty far exceeding any just terrestrial penalty, and has always acknowledged the principled legitimacy of the death penalty.  This is not to say that any grave penalty should lightly be imposed, nor that there may not be general reasons of prudential restraint--although as prudential these may vary, not least because deterrence is a valid further end of just penalty; but it is to say that grave penalty, including penalty of death for the gravest crimes, is not intrinsically evil or invalid.