Objection 1. It would seem that hatred of one's neighbor is not always a sin. For no sin is commanded or counselled by God, according to Proverbs 8:8: "All My words are just, there is nothing wicked nor perverse in them." Now, it is written (Luke 14:26): "If any man come to Me, and hate not his father and mother... he cannot be My disciple." Therefore hatred of one's neighbor is not always a sin.
Objection 2. Further, nothing wherein we imitate God can be a sin. But it is in imitation of God that we hate certain people: for it is written (Romans 1:30): "Detractors, hateful to God." Therefore it is possible to hate certain people without committing a sin.
Objection 3. Further, nothing that is natural is a sin, for sin is a "wandering away from what is according to nature," according to Damascene (De Fide Orth. ii, 4,30; iv, 20). Now it is natural to a thing to hate whatever is contrary to it, and to aim at its undoing. Therefore it seems that it is not a sin to hate one's I enemy.
On the contrary, It is written (1 John 2:9): "He that... hateth his brother, is in darkness." Now spiritual darkness is sin. Therefore there cannot be hatred of one's neighbor without sin.
I answer that, Hatred is opposed to love, as stated above (I-II, 29, 2); so that hatred of a thing is evil according as the love of that thing is good. Now love is due to our neighbor in respect of what he holds from God, i.e. in respect of nature and grace, but not in respect of what he has of himself and from the devil, i.e. in respect of sin and lack of justice.
Consequently it is lawful to hate the sin in one's brother, and whatever pertains to the defect of Divine justice, but we cannot hate our brother's nature and grace without sin. Now it is part of our love for our brother that we hate the fault and the lack of good in him, since desire for another's good is equivalent to hatred of his evil. Consequently the hatred of one's brother, if we consider it simply, is always sinful.
Reply to Objection 1. By the commandment of God (Exodus 20:12) we must honor our parents--as united to us in nature and kinship. But we must hate them in so far as they prove an obstacle to our attaining the perfection of Divine justice.
Reply to Objection 2. God hates the sin which is in the detractor, not his nature: so that we can hate detractors without committing a sin.
Reply to Objection 3. Men are not opposed to us in respect of the goods which they have received from God: wherefore, in this respect, we should love them. But they are opposed to us, in so far as they show hostility towards us, and this is sinful in them. On this respect we should hate them, for we should hate in them the fact that they are hostile to us.
Source: Summa Theologica (New Advent)