Let every person be subordinate to the higher authorities, for there is no authority except from God, and those that exist have been established by God. Therefore, whoever resists authority opposes what God has appointed, and those who oppose it will bring judgment upon themselves. For rulers are not a cause of fear to good conduct, but to evil. Do you wish to have no fear of authority? Then do what is good and you will receive approval from it, for it is a servant of God for your good. But if you do evil, be afraid, for it does not bear the sword without purpose; it is the servant of God to inflict wrath on the evildoer. (Rom 13:1-4)

[Exceptions to the Fifth Commandment of the Decalogue] With regard to the prohibitory part, it should first be taught what kinds of killing are not forbidden by this Commandment. It is not prohibited to kill animals; for if God permits man to eat them, it is also lawful to kill them. When, says St. Augustine, we hear the words, ‘Thou shalt not kill,’ we do not understand this of the fruits of the earth, which are insensible, nor of irrational animals, which form no part of human society. Another kind of lawful slaying belongs to the civil authorities, to whom is entrusted power of life and death, by the legal and judicious exercise of which they punish the guilty and protect the innocent. The just use of this power, far from involving the crime of murder, is an act of paramount obedience to this Commandment which prohibits murder. The end of the Commandment is the preservation and security of human life. Now the punishments inflicted by the civil authority, which is the legitimate avenger of crime, naturally tend to this end, since they give security to life by repressing outrage and violence. Hence these words of David: In the morning I put to death all the wicked of the land, that I might cut off all the workers of iniquity from the city of the Lord. (Catechism of Trent, no. 3500)

Q: Are there cases in which it is lawful to kill?
A: It is lawful to kill when fighting in a just war; when carrying out by order of the Supreme Authority a sentence of death in punishment of a crime; and, finally, in cases of necessary and lawful defense of one’s own life against an unjust aggressor. (Catechism of Saint Pius X, The Fifth Commandment, Question 3 – Spanish)

The State’s effort to contain the spread of behaviors injurious to human rights and the fundamental rules of civil coexistence corresponds to the requirement of watching over the common good. Legitimate public authority has the right and duty to inflict penalties commensurate with the gravity of the crime. the primary scope of the penalty is to redress the disorder caused by the offense. When his punishment is voluntarily accepted by the offender, it takes on the value of expiation. Moreover, punishment, in addition to preserving public order and the safety of persons, has a medicinal scope: as far as possible it should contribute to the correction of the offender. (cf. Lk 23:40-43). The traditional teaching of the Church does not exclude, presupposing full ascertainment of the identity and responsibility of the offender, recourse to the death penalty, when this is the only practicable way to defend the lives of human beings effectively against the aggressor. If, instead, bloodless means are sufficient to defend against the aggressor and to protect the safety of persons, public authority should limit itself to such means, because they better correspond to the concrete conditions of the common good and are more in conformity to the dignity of the human person. (Catechism of the Catholic Church, no. 2266-2267)

Clearly, divine law, both that which is known by the light of reason and that which is revealed in Sacred Scripture, strictly forbids anyone, outside of public cause, to kill or wound a man unless compelled to do so in self defense. (Leo XIII. Encyclical Pastoralis Officii, to the Archbishops and Bishops of the German Empire and Austria-Hungary, September 12, 1881)

This is the context in which to place the problem of the death penalty. On this matter there is a growing tendency, both in the Church and in civil society, to demand that it be applied in a very limited way or even that it be abolished completely. The problem must be viewed in the context of a system of penal justice ever more in line with human dignity and thus, in the end, with God’s plan for man and society. The primary purpose of the punishment which society inflicts is ‘to redress the disorder caused by the offence’ (Catechism of the Catholic Church, No. 2266). Public authority must redress the violation of personal and social rights by imposing on the offender an adequate punishment for the crime, as a condition for the offender to regain the exercise of his or her freedom. In this way authority also fulfils the purpose of defending public order and ensuring people’s safety, while at the same time offering the offender an incentive and help to change his or her behaviour and be rehabilitated (Cf. Catechism of the Catholic Church, No. 2266). It is clear that, for these purposes to be achieved, the nature and extent of the punishment must be carefully evaluated and decided upon, and ought not go to the extreme of executing the offender except in cases of absolute necessity: in other words, when it would not be possible otherwise to defend society. (John Paul II. Encyclical Evangelium Vitae, no. 56, March 25, 1995)

Even when it is a question of the execution of a condemned man, the State does not dispose of the individual’s right to life. In this case it is reserved to the public power to deprive the condemned person of the enjoyment of life in expiation of his crime when, by his crime, he has already disposed himself of his right to live. (Pius XII. Address to the participants in the First International Congress on Histopathology of the Nervous System, no. 33, September 14, 1952)

However, there are some exceptions made by the divine authority to its own law, that men may not be put to death. These exceptions are of two kinds, being justified either by a general law, or by a special commission granted for a time to some individual. And in this latter case, he to whom authority is delegated, and who is but the sword in the hand of him who uses it, is not himself responsible for the death he deals. And, accordingly, they who have waged war in obedience to the divine command, or in conformity with His laws, have represented in their persons the public justice or the wisdom of government, and in this capacity have put to death wicked men; such persons have by no means violated the commandment, ‘You shall not kill.’ (Saint Augustine. City of God, Book I, Ch. 21)

But great and holy men, although they at the time knew excellently well that that death which separates the soul from the body is not to be dreaded, yet, in accordance with the sentiment of those who might fear it, punished some sins with death, both because the living were struck with a salutary fear, and because it was not death itself that would injure those who were being punished with death, but sin, which might be increased if they continued to live. They did not judge rashly on whom God had bestowed such a power of judging. Hence it is that Elijah inflicted death on many, both with his own hand and by calling down fire from heaven; as was done also without rashness by many other great and godlike men, in the same spirit of concern for the good of humanity. (Saint Augustine. On the Sermon on the Mount, Book I, Ch. 20, no.64)

Conversely, the power of the sovereign, the right over life and death of the judge, the iron hook of the executioner, the soldier’s weapon, the power to punish of the authority, and even the severity of the good father have not been instituted in vain. All these regulations have their measure, their causes, their reasons and their utility. When these are feared, not only are the evil refrained, but even the good live more tranquilly among the evil. […] It is not useless to repress the arrogance and prepotency of men even by the fear of human laws, so that not only innocence might have security among the wicked, but also so that these same wicked ones may have, in fear of a torment, a brake to their possibility of doing evil, and so may they invoke God to cure their will to do so. (Saint Augustine. Letter to Macedonius, no.153, Ch. 6, no. 16)

The slaying of a man is forbidden in the decalogue, in so far as it bears the character of something undue: for in this sense the precept contains the very essence of justice. Human law cannot make it lawful for a man to be slain unduly. But it is not undue for evil-doers or foes of the common weal to be slain: hence this is not contrary to the precept of the decalogue; and such a killing is no murder as forbidden by that precept, as Augustine observes (De Lib. Arb. I, 4). In like manner when a man’s property is taken from him, if it be due that he should lose it, this is not theft or robbery as forbidden by the decalogue. (Saint Thomas Aquinas. Summa Theologica, I-II, q. 100, a. 8, ad. 3)

Now every part is directed to the whole, as imperfect to perfect, wherefore every part is naturally for the sake of the whole. For this reason we observe that if the health of the whole body demands the excision of a member, through its being decayed or infectious to the other members, it will be both praiseworthy and advantageous to have it cut away. Now every individual person is compared to the whole community, as part to whole. Therefore if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and advantageous that he be killed in order to safeguard the common good, since ‘a little leaven corrupteth the whole lump’ (1Cor 5:6). (Saint Thomas Aquinas. Summa Theologica, II-II, q.64, a.2)

According to the order of His wisdom, God sometimes slays sinners forthwith in order to deliver the good, whereas sometimes He allows them time to repent, according as He knows what is expedient for His elect. This also does human justice imitate according to its powers; for it puts to death those who are dangerous to others, while it allows time for repentance to those who sin without grievously harming others. (Saint Thomas Aquinas. Summa Theologica, II-II, q.64, a.2, ad 2)

It is for this reason that both Divine and human laws command such like sinners to be put to death, because there is greater likelihood of their harming others than of their mending their ways. Nevertheless the judge puts this into effect, not out of hatred for the sinners, but out of the love of charity, by reason of which he prefers the public good to the life of the individual. (Saint Thomas Aquinas. Summa Theologica, II-II, q.25, a.6, ad 2)