...when insulting him? Why do they call him "Hussein Osama" and things like that? I can understand if they don't like his policies, but why are they trying to imply that he's an "evil Muslim"?

First of all, Obama was never a Muslim though he has been around Muslims.

But more importantly, what's wrong with Muslims? Just because .001% are crazy jihadist terrorists doesn't mean 99.99% of them are.