Thomas Aquinas

“ I answer that, Discord denotes a certain disunion of wills, in so far, to wit, as one man's will holds fast to one thing, while the other man's will holds fast to something else. Now if a man's will holds fast to its own ground, this is due to the act that he prefers what is his own to that which belongs to others, and if he do this inordinately, it is due to pride and vainglory. Therefore discord, whereby a man holds to his own way of thinking, and departs from that of others, is reckoned to be a daughter of vainglory. ”

“ It follows from this that discord is sometimes the sin of one party only, for instance, when one wills a good which the other knowingly resists; while sometimes it implies sin in both parties, as when each dissents from the other's good, and loves his own.
Reply to Objection 1: One man's will considered in itself is not the rule of another man's will; but in so far as our neighbor's will adheres to God's will, it becomes in consequence, a rule regulated according to its proper measure. ”

“ Whether it is a daughter of vainglory?
Art. 1 - Whether discord is a sin? Objection 1: It would seem that discord is not a sin. For to disaccord with man is to sever oneself from another's will. But this does not seem to be a sin, because God's will alone, and not our neighbor's, is the rule of our own will. Therefore discord is not a sin. ”