Zimmerman had ability to retreat before the confrontation. He had no _duty_ to do it (that's SYG law). We might never know what happened, perhaps Zimmerman called Trayvon 'a fucking n...er' or threatened to shoot him on the spot.
Imagine that I walk into a bar and start a brawl with the biggest baddest meanie there, then I pull out my gun and shoot him. It appears to be totally legal under Florida's laws (well, I can be nicked for getting a weapon into a bar - that's a felony, btw). Should I be let go in this case? Now imagine that there is nobody in the bar at that time, so it's my word against corpse's. I'll even have bruises to prove that I was threatened!
That's why SYG laws are dumb and should be repealed. Civilians should certainly be able to use deadly force in self-defense (or in defense of other peoples' lives), or to protect their property ('castle doctrine') but that's it.