Proverbs 10:18

18 He who conceals his hatred has lying lips,
and whoever spreads slander is a fool.

So if I hate someone, I am not supposed to hold in it, according to this verse. But then the verse also says that I am not supposed to let it out by slandering the person that I hate behind his back. That leaves one option right? I must tell that person how much I hate him to his face. Of course not! There is only one other option and that would be the correct one-- don't hate him at all! I think the point is, there is no way we can act appropriately on our hate-- so don't do it.