I love having KG on this team, but I don't think it's very hard to understand why he is hated.
http://www.youtube.com/watch?v=M5n5WNmzfOo&feature=relatedHomervision: Oh that was so great! KG is so intense.
Opponentvision: What is this clown doing? what a jerk

Homervision: Man KG is intense.
Opponentvision: Get this @#! off the court he's not even playing!
Oh and the trash talking... heres what Spike Lee had to say.
So look, I don't like the Celtics, but I respect Doc [Rivers]. Doc's a good friend. You know I love Ray Allen. I hate the Celtics, but look, they've got good guys on that team. I've got friends on that team. Nate’s on the team. But Garnett needs to calm the f--- down. There's no reason he should be cursing at me the way he did the last game. So you can put that in the article. It was disrespectful and I would never do nothing like that to him.
I've heard that KG goes pretty far with the trash talk from someone who has been to a few games sitting courtside. He doesn't pull punches and he aims to hurt. And it seems that it's not directed exlusively at players.
Personally I have no problem with KG doing or saying what he does on the court, but lets not act like it's some big mystery why he's hated.