Objection 1. It would seem that hatred of our neighbor is the most grievous sin against our neighbor. For it is written (1 John 3:15): "Whosoever hateth his brother is a murderer." Now murder is the most grievous of sins against our neighbor. Therefore hatred is also.
Objection 2. Further, worst is opposed to best. Now the best thing we give our neighbor is love, since all other things are referable to love. Therefore hatred is the worst.
On the contrary, A thing is said to be evil, because it hurts, as Augustine observes (Enchiridion xii). Now there are sins by which a man hurts his neighbor more than by hatred, e.g. theft, murder and adultery. Therefore hatred is not the most grievous sin.
Moreover, Chrysostom [Hom. x in the Opus Imperfectum, falsely ascribed to St. John Chrysostom] commenting on Matthew 5:19, "He that shall break one of these least commandments," says: "The commandments of Moses, Thou shalt not kill, Thou shalt not commit adultery, count for little in their reward, but they count for much if they be disobeyed. On the other hand the commandments of Christ such as, Thou shalt not be angry, Thou shalt not desire, are reckoned great in their reward, but little in the transgression." Now hatred is an internal movement like anger and desire. Therefore hatred of one's brother is a less grievous sin than murder.
I answer that, Sins committed against our neighbor are evil on two counts; first by reason of the disorder in the person who sins, secondly by reason of the hurt inflicted on the person sinned against. On the first count, hatred is a more grievous sin than external actions that hurt our neighbor, because hatred is a disorder of man's will, which is the chief part of man, and wherein is the root of sin, so that if a man's outward actions were to be inordinate, without any disorder in his will, they would not be sinful, for instance, if he were to kill a man, through ignorance or out of zeal for justice: and if there be anything sinful in a man's outward sins against his neighbor, it is all to be traced to his inward hatred.
On the other hand, as regards the hurt inflicted on his neighbor, a man's outward sins are worse than his inward hatred. This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)