Innocent until proven guilty - only he already admitted he did the act. Now it's just a matter of determining the legality of the act. (That's the issue with the whole "affirmative defense" - once you admit you did it, it actually becomes guilty until proven innocent. The burden moves to YOU to prove what you did was justified.)
We have a few facts:
1. George Zimmerman followed Trayvon Martin, after being told not to by 911 dispatchers.
2. George Zimmerman and Trayvon Martin got into a fight.
3. George Zimmerman shot Trayvon Martin dead.
We know, after the fact, that Martin was innocent of any wrongdoing in walking around, and was unarmed. Obviously, Zimmerman could still have been "in fear of his life", but after-the-fact we know that the cause for the shooting was not justified. But, based on the circumstances between fact 2 and fact 3, was the shooting itself justified? That's what he has to prove. The fact that his head was bloodied only continues to provide evidence that they were in a fight, not who started it. If Zimmerman started it, his getting his head bloodied may very well have been valid self-defense actions from Martin!
When I took my CHL class, it was made very clear that in both my home state and per Utah law, if you escalate the situation, you lose self-defense as an affirmative defense. To me, it certainly appears that because of fact 1, Zimmerman escalated and therefore should lose the affirmative defense.
Hopefully there is a witness of what happened between fact 1 and fact 2 that can truly clear up any ambiguities about how the confrontation started. Did Martin suddenly turn around and rush Zimmerman? To me, even though Zimmerman "escalated" by following, that would make the shooting justified. Did Zimmerman yell at Martin to stop, and a verbal argument turned physical? That would make it a tough call, but I would argue that would make it unjustified to me (although not "Murder" level - probably a lower Manslaughter.) Did Zimmerman pull his gun before the fight, and Martin responded to the gun being pulled by attacking? Totally unjustified, definitely Murder.
We won't know for sure until/unless a witness comes forward.
