Objection 1: It seems unlawful to kill people who have sinned. For our Lord forbade, in the parable (Mt 13), the pulling up of the tares, which according to a gloss signifies wicked people. Now, whatever is forbidden by God is a sin. Therefore, it is a sin to kill a sinner.
Objection 2: Further, human justice is modeled after divine justice. Now, according to divine justice, sinners are kept for repentance, as stated in Ez 33:11: „I desire not the death of the wicked, but that the wicked turn from his way and live.“ Therefore, it seems altogether unjust to kill sinners.
Objection 3: Further, it is not permitted for any good purpose to do what is evil in itself, according to Augustine (Contra Mendac. vii) and the Philosopher (Ethic. ii, 6). Now, it is evil in itself to kill a man, since we are bound to have charity towards all men, and „we wish our friends to live and exist,“ according to Ethic. ix, 4. Therefore, it is by no means permitted to kill a man who has sinned.
On the contrary, it is written (Ex 22:18): „You shall not suffer a sorcerer to live“; and (Ps 101:8): „In the morning I put to death all the wicked of the land.“
I answer that: As stated above (Article 1), it is lawful to kill irrational animals in so far as they are naturally ordered to man’s use, as the imperfect is ordered to the perfect. Now, every part is ordered to the whole, as the imperfect to the perfect; wherefore every part exists 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 because it is decayed or infectious to the others, it is both praiseworthy and advantageous to cut it off. Now, every individual man stands in relation to the whole community as a part to the whole. Therefore, if a man be dangerous and infectious to the community, on account of some sin, it is praiseworthy and salutary to kill him in order to safeguard the common good, since „a little leaven corrupts the whole lump“ (1 Cor 5:6).
Reply to Objection 1: Our Lord forbade them to pull up the tares, lest the wheat, that is, the good, should be rooted up with them. This happens when the wicked cannot be slain without danger to the good, either because the wicked are hidden among the good, or because they have many followers, so that they cannot be slain without danger to the good, as Augustine says (Contra Parmen. iii, 2). Therefore, our Lord teaches that the wicked should be allowed to live, and that vengeance should be deferred until the last judgment, rather than that the good be slain together with the wicked. But when there is no such danger to the good, but rather they are protected and saved by the slaying of the wicked, then the latter may be lawfully put to death.
Reply to Objection 2: According to the order of His wisdom, God sometimes at once strikes sinners dead, in order to deliver the good, while at other times He gives them time to repent, as He knows what is expedient for His elect. This also human justice imitates as far as it can; for it puts to death those who are dangerous to others, while allowing time for repentance to those who sin without grievous harm to others.
Reply to Objection 3: By sinning, man departs from the order of reason and so 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, so that he may be disposed of according as is useful to others. This is expressed in Ps 49:21: „Man, when he was in honor, did not understand; he has been compared to senseless beasts, and made like to them,“ and Prov 11:29: „The fool shall serve the wise.“ Therefore, although it is evil in itself to kill a man so long as he preserves his dignity, yet it may be good to kill a man who has sinned, just as it is good to kill a beast. For a wicked man is worse and more harmful than a beast, as the Philosopher notes (Polit. i, 1 and Ethic. vii, 6).