That isn't self defense.
That is true. However, it is not up to Zimmerman to prove that he didn't start the fight - it is up to the prosecution to prove beyond a reasonable doubt that Zimmerman started the fight in order to negate any self-defense claim by Zimmerman. So far, the evidence appears to point toward Martin being the aggressor, but the state probably cannot prove who started the fight one way or another. In this case, that favors Zimmerman, because legally, he doesn't have to prove anything.
The State has one heck of a case in front of them and a high hurdle of evidence to clear - because so far there have been no reports of any witnesses to who initiated the fight. We have the interview of George after the fact - and Treyvon - the only other witness - is dead.
I am sure much hay will be made of - and straw attempted to be spun into gold with - the ‘not getting away again’ quote from George - and the ‘you don't need to do that (follow him)’ from the dispatcher.
What evidence points towards Martin being the aggressor?