first, what proof do you have he murdered trayvon?
We have "he said, dead guy said" cases all of the time - and believe it or not, people ARE convicted of manslaughter or murder even without witnesses to the crime.
Toss in zimm's tape saying trayvon was running away... and he admits he's pursuing him...
It just doesn't look good, I'd say. Sure, nobody saw the shooting... but we know who was using profanity and pursuing, and who was running away. Jury is going to look at that and say "to believe trayvon started the fight, I have to create a premise in my head that says Trayvon went from running wimp to angry pothead attack machine - while simultaneously zimerman ended his profanity-fueled sprint to catch the alleged bike thief.
I just don't see BOTH of those happening simultaneously. Do you? And none of us can prove a negative. "Prove aliens didn't freeze zimmerman while trayvon attacked him...."