Honestly, I think it's okay to hate some people. I have had people in my life who, through their actions towards me, deserve none of my respect anymore. Their actions were atrocious and unforgivable. I'm not the type to "forgive and forget" because some things should not be.
That said, I don't think it's possible to hate somebody without having loved them to begin with. Or having loved a concept related to them. For there to be hatred there has to be a love for someone orsomething behind the hatred - otherwise why would we care so much to put so much passion behind dislike?
So to sum up, I guess you could say I only am able to truly hate somebody that I once loved almost as equally. Really quite depressing to think about.