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.

❌ Objection 2 : Further, as stated above (Article [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.

❌ Objection 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 (Article [2]). Therefore it is deserving of praise if even private individuals kill evil-doers.