But how relevant is that, really. Regardless of who initiated the confrontation, the facts are that you have Martin on top of Zimmerman beating the crap out of him, with Zimmerman yelling for help, which means his life was in danger at that point. The only thing you can argue now is that Martin was justified and acting in self defense himself, of which, no evidence exists. In fact, the very position that Martin finds himself in...on top of Zimmerman...doesn't speak well for a self defense motive. You would have to further argue that Martin found it necessary to incapacitate Zimmerman since Martin was in control of situation.
That seems to be a pretty heavy burden to prove.
Leaving the above aside, let's say Martin was on top. You state that he was in control of the situation! Was he? How much in control can someone who is unarmed be against someone who is armed? In your scenereo I'd have to argue there is a case of self-defense that can be made for Martin as he was unarmed, and fighting an armed man. I don't know about you, but if I found myself in that position I wouldn't stop hitting the other person until I knew he waw either no longer armed or completely incapacitated.