It's baffling how much hate Lesnar has gotten. I attribute most of it to bitter Fedor fanboys, but you're right, he was hated from the start. Jealousy I'm guessing.
What's baffling about it? He came into the UFC expecting a silver platter. He said so himself; he thinks he's better than everyone and he's not afraid to say it. Nevermind his incredible ability or phenomenal physique, he's pompous and THIS has rubbed people the wrong way.
I'm sure a lot of people were on the fence about him until his rant after beating Mir. It's hard to cheer for someone like that. He's very new to the sport and to those who say no one respects him, he shouldn't expect more than he's given.
My two cents. That being said, I hope his career doesn't end due to illness and that he can continue in the UFC. He's been fairly entertaining up to now, and I'm sure he's got some great fights ahead of him. He's kind of like a much bigger Tito... but not funny.
And to whoever said he's made himself the bad guy, that pretty much summed it up. He is his own creation.