Whether it is lawful to kill a man in self-defense?

❌ Objection 1 : It would seem that nobody may lawfully kill a man in self-defense. For Augustine says to Publicola (Ep. xlvii): "I do not agree with the opinion that one may kill a man lest one be killed by him; unless one be a soldier, exercise a public office, so that one does it not for oneself but for others, having the power to do so, provided it be in keeping with one's person." Now he who kills a man in self-defense, kills him lest he be killed by him. Therefore this would seem to be unlawful.

❌ Objection 2 : Further, he says (De Lib. Arb. i, 5): "How are they free from sin in sight of Divine providence, who are guilty of taking a man's life for the sake of these contemptible things?" Now among contemptible things he reckons "those which men may forfeit unwillingly," as appears from the context (De Lib. Arb. i, 5): and the chief of these is the life of the body. Therefore it is unlawful for any man to take another's life for the sake of the life of his own body.

❌ Objection 3 : Further, Pope Nicolas [*Nicolas I, Dist. 1, can. De his clericis] says in the Decretals: "Concerning the clerics about whom you have consulted Us, those, namely, who have killed a pagan in self-defense, as to whether, after making amends by repenting, they may return to their former state, or rise to a higher degree; know that in no case is it lawful for them to kill any man under any circumstances whatever." Now clerics and laymen are alike bound to observe the moral precepts. Therefore neither is it lawful for laymen to kill anyone in self-defense.

❌ Objection 4 : Further, murder is a more grievous sin than fornication or adultery. Now nobody may lawfully commit simple fornication or adultery or any other mortal sin in order to save his own life; since the spiritual life is to be preferred to the life of the body. Therefore no man may lawfully take another's life in self-defense in order to save his own life.

❌ Objection 5 : Further, if the tree be evil, so is the fruit, according to Mt. 7:17. Now self-defense itself seems to be unlawful, according to Rm. 12:19: "Not defending [Douay: 'revenging'] yourselves, my dearly beloved." Therefore its result, which is the slaying of a man, is also unlawful.