In addition, the autopsy found the bullt hole. along with powder burns as well as trauma to Martin's knuckles. No facial injury to indicate Zimmerman hit him first as per your claim.
Even if it were the case that Zimmerman started the fight, once Martin had him on the ground and Zimmerman was calling for help, Martin would have the obligation to cease the pummelling. If he didn't Zimmerman, would be perfectly within his right to shoot him since he could not retreat with Martin on top of him.
In the 7-11 video, after taking forever to pay for his goodies, he wanders aimlessly in the opposite direction of the exit, then turns and wanders back toward the front of the store and the door to outside. I'd say he was a bit confused as he looked like he didn't know where the exit from the store was.