I simply don't understand how who started the altercation has any relevance at all (Phantom Wolf has said this often as well). Are people saying that if Martin took a swing at Zimmerman, Z was justified in killing him? Crap. Not that simple at all.
It's not simply who started the fight, though.
In one scenerio, Zimmerman is following Trayvon and Trayvon is walking/running away. If that is the case than Zimmerman is not "defending" himself from Trayvon because Trayvon is not posing a threat by fleeing. In that scenerio, even if Trayvon threw the first punch, it would be Trayvon acting in self defense as Zimmerman's actions can be considered threatening.
In the other scenerio, Zimmerman did get out of the truck to follow but did decide to go back to the truck. In that scenerio, even if Zimmerman was at one point, a threat, his is no longer a threat because he now fleeing Trayvon. Trayvon now becomes the threat, following Zimmerman. Now, if Trayvon throws the first punch, after approaching a fleeing individual, it is assault and Zimmerman is defending himself.
There are, of course, other factors. I believe the prosecution is going with Zimmerman pursuing Martin and starting the fight, with Martin being the one screaming for help, and Zimmerman pulls out his gun and shoots. If that were the case, then it is clearly 2nd degree murder.
If they can only convince the jury that Zimmerman continued to follow Martin and it is unclear who started the fight but the end result was that Zimmerman shot Martin, it's one of the manslaughters (I forget which one), because Zimmerman, regardless of his intent, caused a wrongful death.