Do you have evidence or proof that Z started the fight? Wounds to Martin that would show this? I think this is the main problem with people making this story up in their head - there is ZERO proof that Z initiated physical contact or the fight.
Are you saying that if Martin did initiate they fight/assault then Zimmerman was justified in shooting as he was on his back being beaten and crying for help?
I see no evidence that Zimmerman started the fight, but Alkemyst's post of 9-1-1 transcripts apparently pokes a hole in one part of his story. If I understand it correct, Zimmerman claimed that Martin attacked him from behind while he was walking back to his truck after getting out to see what street he was on. Yet at least a couple calls indicated that the fight was "behind my house". I'd have to see the area on a map to be sure, but "behind my house" doesn't seem like where one would be after getting out of the truck to check a street sign. (I admit it is still possible; sometimes houses back not to other houses, but to a road, where there isn't enough room between roads for two rows of houses. But it doesn't seem likely.) It seems much more likely that Zimmerman chased Martin in his truck, then when the kid cut between houses pursued him on foot. At that point a physical fight started. I'd guess that Martin got tired of running and started it, but we don't actually no that; he might have run into a fenced area with no escape, at which point Zimmerman grabbed him to detain him for the police. We really have no way to tell at this point. I can easily come up with plausible theories either way as to who initiated a physical confrontation, and the fact that an athletic but skinny 6'-8" 17 year old got the best of an overweight man a decade his senior isn't necessarily evidence that the kid started it.
For that matter, it's entirely possible that Martin beat him down, then got up, thinking the fight over, at which point Zimmerman was free to pull his gun and shoot Martin. We simply don't know. We may never know.
The only thing we know for sure is that an innocent 17 year old kid is dead from a confrontation that never should have happened. No matter how the actual thing went down, that is incontrovertible.