Objection 1. It would seem that strife is not always a sin. For strife seems a kind of contention: hence Isidore says (Etym. x) that the word "rixosus [quarrelsome] is derived from the snarling [rictu] of a dog, because the quarrelsome man is ever ready to contradict; he delights in brawling, and provokes contention." Now contention is not always a sin. Neither, therefore, is strife.
Objection 2. Further, it is related (Genesis 26:21) that the servants of Isaac "digged" another well, "and for that they quarrelled likewise." Now it is not credible that the household of Isaac quarrelled publicly, without being reproved by him, supposing it were a sin. Therefore strife is not a sin.
Objection 3. Further, strife seems to be a war between individuals. But war is not always sinful. Therefore strife is not always a sin.
On the contrary, Strifes [The Douay version has 'quarrels'] are reckoned among the works of the flesh (Galatians 5:20), and "they who do such things shall not obtain the kingdom of God." Therefore strifes are not only sinful, but they are even mortal sins.
I answer that, While contention implies a contradiction of words, strife denotes a certain contradiction of deeds. Wherefore a gloss on Galatians 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. On 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; 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.
Reply to Objection 1. Strife is not just the same as contention: and there are three things in the passage quoted from Isidore, which express the inordinate nature of strife. First, the quarrelsome man is always ready to fight, and this is conveyed by the words, "ever ready to contradict," that is to say, whether the other man says or does well or ill. Secondly, he delights in quarrelling itself, and so the passage proceeds, "and delights in brawling." Thirdly, "he" provokes others to quarrel, wherefore it goes on, "and provokes contention."
Reply to Objection 1. The sense of the text is not that the servants of Isaac quarrelled, but that the inhabitants of that country quarrelled with them: wherefore these sinned, and not the servants of Isaac, who bore the calumny [Cf. Genesis 26:20.]
Reply to Objection 3. In order for a war to be just it must be declared by authority of the governing power, as stated above (Question 40, Article 1); whereas strife proceeds from a private feeling of anger or hatred. For if the servants of a sovereign or judge, in virtue of their public authority, attack certain men and these defend themselves, it is not the former who are said to be guilty of strife, but those who resist the public authority. Hence it is not the assailants in this case who are guilty of strife and commit sin, but those who defend themselves inordinately.
Source: Summa Theologica (New Advent)