Thomas Aquinas

“ 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. ”

“ Wherefore a gloss on Gal. 5:20 says that "strifes are when persons strike one another through anger." Hence strife is a kind of private war, because it takes place between private persons, being declared not by public authority, but rather by an inordinate will. Therefore strife is always sinful. In fact it is a mortal sin in the man who attacks another unjustly, for it is not without mortal sin that one inflicts harm on another even if the deed be done by the hands. But in him who defends himself, it may be without sin, or it may sometimes involve a venial sin, or sometimes a mortal sin ”

“ Boasting and puffing up of self which are the result of anger or vainglory, are not the direct but the occasional cause of quarrels or strife, because, when a man resents another being preferred to him, his anger is aroused, and then his anger results in quarrel and strife. ”