I agree that hate is most harmful to the one harboring such an emotion. Negative emotions like envy, jealousy, hate, suspicion and hence any amount of grudges are like placing poison inside of your mind. However, to say that one can rid oneself of hate completely and be a loving being who loves everyone and everything...in my opinion, is a fatuous claim. Hate is the reciprocal of love and where there is love, there is hate.
The world is filled with a drastic variety of people, and we all have some principles, whether we conform to a religion or not. There will be people who do not conform to your beliefs and you learn to tolerate them, accept them as individuals who have made their choices. There will be people who work against you and your ideals, significantly harming material or abstract things significant to you or other human lives.
Can I love Hitler? No, frankly I can't. I can accept that the various theories about him being a sociopath, a schizophrenic and whatnot are true and like most psychopaths/serial killers, he's simply a sick guy who ended up doing horrible things out of his sickness, I still cannot love him because what he did to at least a couple of generations of the European and US community cannot be just ignored.
I'm not saying that one should go around tossing grenades at German people, because that's just plain stupid, but I believe in standing for your principles, defining them and protecting them. It's who I am and I can't deny it just because someone will have to look bad in my head. Hate is a very strong word and I've only used it for a few people, and never for people I know personally--because yes, it's a harmful poison. Just like war and any other aggressive state of mind or affairs. But you don't stop vouching for it simply because it can be detrimental; it serves bigger purposes and speaks for your values. At least, that's my take on hate.