Summa Theologica, Secunda Secundae Partis[/i], Q. 41, Art. 1]But in him who defends himself, it may be without sin, or it may sometimes involve a venial sin, or sometimes a mortal sin; and this depends on his intention and on his manner of defending himself. For if his sole intention be to withstand the injury done to him, and he defend himself with due moderation, it is no sin, and one cannot say properly that there is strife on his part. But if, on the other hand, his self-defense be inspired by vengeance and hatred, it is always a sin. It is a venial sin, if a slight movement of hatred or vengeance obtrude itself, or if he does not much exceed moderation in defending himself: but it is a mortal sin if he makes for his assailant with the fixed intention of killing him, or inflicting grievous harm on him.
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