Catholic Encyclopedia (1913)/Unjust Aggressor
Aggressor, Unjust.—According to the accepted teaching of theologians, it is lawful, in the defence of life or limb, of property of some importance, and of chastity, to repel violence with violence, even to the extent of killing an unjust assailant. This is admitted to be true with the reservation included in the phrase "servato moderamine inculpatæ tutelæ." That is, only that degree of violence may be employed which is necessary adequately to protect one from the attack. For example, if it were enough in the circumstances to maim an enemy it would be unlawful to kill him. It is likewise lawful to aid another to the same extent and within the same limits as are permissible for self-defence. (See Homicide.)
Gury, Comp. Theol. Moral. (Prato, 1901) I, 381; Liguori, n. 380.