Hate the Actions, Not The Person

Even if you have a "legitimate" reason to hate someone, such as they behave completely contrary to the Torah's commandments (the commentaries say that only very great people should employ such feelings), those feelings will lead to a pure "hatefest" where both parties develop illegitimate negative feelings toward each other.  The best thing you can do is overcome your legitimate negative feelings and treat him nicely by even doing favors.

This may be why the Torah commands us to help our enemy whose donkey is overburdened.

"If you see the donkey of someone you hate crouching under its burden... you must help him." Exodus 23:5