HKB
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.
Reply Obj. 3: By sinning man departs from the order of reason, and consequently falls away from the dignity of his manhood, in so far as he is naturally free, and exists for himself, and he falls into the slavish state of the beasts, by being disposed of according as he is useful to others. This is expressed in Ps. 48:21: "Man, when he was in honor, did not understand; he hath been compared to senseless beasts, and made like to them," and Prov. 11:29: "The fool shall serve the wise." Hence, although it be evil in itself to kill a man so long as he preserve his dignity, yet it may be good to kill a man who has sinned, even as it is to kill a beast. For a bad man is worse than a beast, and is more harmful, as the Philosopher states (Polit. i, 1 and Ethic. vii, 6).
THIRD ARTICLE [II-II, Q. 64, Art. 3]
Whether It Is Lawful for a Private Individual to Kill a Man Who Has Sinned?
Objection 1: It would seem lawful for a private individual to kill a man who has sinned. For nothing unlawful is commanded in the Divine law. Yet, on account of the sin of the molten calf, Moses commanded (Ex. 32:27): "Let every man kill his brother, and friend, and neighbor." Therefore it is lawful for private individuals to kill a sinner.
Obj. 2: Further, as stated above (A. 2, ad 3), man, on account of sin, is compared to the beasts. Now it is lawful for any private individual to kill a wild beast, especially if it be harmful. Therefore for the same reason, it is lawful for any private individual to kill a man who has sinned.
Obj. 3: Further, a man, though a private individual, deserves praise for doing what is useful for the common good. Now the slaying of evildoers is useful for the common good, as stated above (A. 2). Therefore it is deserving of praise if even private individuals kill evil-doers.
On the contrary, Augustine says (De Civ. Dei i) [*Can. Quicumque percutit, caus. xxiii, qu. 8]: "A man who, without exercising public authority, kills an evil-doer, shall be judged guilty of murder, and all the more, since he has dared to usurp a power which God has not given him."
I answer that, As stated above (A. 2), it is lawful to kill an evildoer in so far as it is directed to the welfare of the whole community, so that it belongs to him alone who has charge of the community's welfare. Thus it belongs to a physician to cut off a decayed limb, when he has been entrusted with the care of the health of the whole body. Now the care of the common good is entrusted to persons of rank having public authority: wherefore they alone, and not private individuals, can lawfully put evildoers to death.
Reply Obj. 1: The person by whose authority a thing is done really does the thing as Dionysius declares (Coel. Hier. iii). Hence according to Augustine (De Civ. Dei i, 21), "He slays not who owes his service to one who commands him, even as a sword is merely the instrument to him that wields it." Wherefore those who, at the Lord's command, slew their neighbors and friends, would seem not to have done this themselves, but rather He by whose authority they acted thus: just as a soldier slays the foe by the authority of his sovereign, and the executioner slays the robber by the authority of the judge.