I would say that following somebody around and confronting them (what, do you think Trayvon Martin stopped walking and yanked him out of his car or something?), solely because they are walking in "your" neighborhood, would fit the definition of "start[ing] the fight." Particularly if you do so with a firearm.
Much as he might like to believe otherwise, this man is not a police officer. He has no right to stop anybody for anything, and in choosing to do so he took the danger upon himself. That's exactly why the dispatcher told him he didn't need to follow the kid: Because doing so could be dangerous, either to the kid or to Zimmerman himself. Turns out it was.
If Martin was armed, then I might be able to believe some kind of self defense claim. He wasn't, and Zimmerman has made no kind of "ZOMG I thought your Skittles were a gun!" claim of a mistaken impression. For that matter, Martin was shot in the back. Precisely what kind of imminent danger are you in from somebody who is not even facing you (and who may have been on the ground at the time depending on how the stories unfolded)?
While we're at it, yeah, we have to address the race question. Have you noticed how every day, Zimmerman's father comes out and yaps to the police about how innocent his son is how many black friends he has with all sorts of details he's either inventing out of thin air or, more likely, he got from his son? If you've followed the case it's hard to miss. So with all this information Zimmerman is essentially leaking, why have we not heard a single word about the whole basis for the confrontation and the catalyst for the reaction thereafter? Why have we not heard a single word about how walking with an iced tea makes you suspicious at all, much less suspicious enough to call the police about and then stalk? It's what everyone on the planet most wants to know about this case and the only thing he doesn't have an answer for. I find that highly suspicious.
Zimmerman is a wanna-be cop tough guy who had a seventeen year old kid show him exactly how fast he craps his pants when confronted with "danger." He deserves what he got and more -- like, say, a nice prison sentence.