Saint Augustine and Saint Thomas Aquinas supported the death penalty

 

Sacred Scripture

The legitimate authority must do justice and chastise the evildoer

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)

St. Augustine and St. Thomas Aquinas Pro death penalty

Saint Augustine of Hippo

Those who, representing public justice, put to death wicked men have by no means violated the commandment ‘You shall not kill’

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)

Great and holy men punished some sins with death

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)

Fear of the law represses the evil and gives security to the good

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)

Saint Thomas Aquinas

The slaying of evil-doers is not contrary to the precept of the Decalogue

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)

The death penalty must be applied to safeguard the common good

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)

Human justice should imitate divine justice

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)

Both Divine and human laws command such sinners to be put to death out of the love of charity

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)