Earth 2160. Which character? ALL OF THEM. Those writers should be shot, revived and shot again.
Space Siege. Well, not really hate, I just don't care about the guy I'm playing. Not really much of a character or personality there. Even the one-dimensional supporting cast is more interesting than Mr. "I'll think about it".
GTA: San Andreas. I hate the whole "gangsta" culture, and I hate every part of San Andreas that takes place in Los Santos. As soon as it goes outside the city, however, I really really love it.
Dawn of War. Well, not really hate, but even the GOOD guys regularly commit mass murder in the name of the Greater Good, and no-one except the T'au can really be called the "good guys" on a regular basis with a straight face. The human factions especially, bunch of genocidal twats. (I've never played as Chaos.)