I'm not as angry about this, because I don't think it's beyond reasonable doubt that Zimmerman was defending himself. Maybe after the altercation started, Trayvon quickly got the upper hand, and decided to go a step further. Maybe he was on top, and said something about teaching Zimmerman a lesson. At which point, Zimmerman feared that Trayvon might kill him. For that matter, Trayvon may have actually intended to do so.
I agree that he's probably guilty, but it's far from a sure thing. What is important is that this went to a trial. Remember, the police were originally not going to so much as charge Zimmerman. That's where the controversy really got going. If they had booked him immediately, I don't think the media would have latched on to the same degree.