Actually,Zimmerman didn't call 9-1-1, he called the police department directly on the non emergency line. Not that it makes a huge difference, but I mention it for accuracy.
We don't know what happened. What we do get from the limited information released so far does seem like Zimmerman thought he was Barney Feiff and confronted Martin. Martin being a 17 year old athlete he was likely far more agressive and physically capable than the overweight 'watch captain'. In short, Zimmermam bit off more than he could chew. At the moment of the shot, he was likely in mortal danger. He got in that danger because he exercised poor judgement and exercised authority he didn't have. A thinking neighborhood watch volunteer would have made the phone call, kept his distance and maybe followed the suspicious individual. Maybe snap a picture. Confronting someone with "What are you doing in my neighborhood?!" shouldn't have earned him a butt whooping....but that's what young Martin gave him.
No fooling, when a body builder takes steroids, the target testoserone blood level is about what a teenager naturally has. This is one reason why teenagers get in more fights that any other age group. Having some chubby toolbag get in his face could be all it took to start the dance. Maybe Martin saw the gun and tried to take it. Maybe he just started smashing head because it was a bad week and this guy opened the whoopass can. Whatever. Once it started, Zimmerman was in big trouble. But it seems from all accounts that it started because Zimmerman initiated contact with a person he was suspicous of. Who gave him the right to go confront anybody? Does the apartment association have that right to delegate? Like I said before, this guy apears to have a serious Barney Feiff complex.
The big question will be, does it matter if Zimmerman 'started' the confrontation? He was clearly in fear for his life when he shot, but did he put himself there? Does that matter?
My prediction is that the outcome of the court case will have more to do with political concearns than with truth or justice. The State of Florida doesn't want to host the next Watts/Rodney King style riots. They will throw Zimmerman under the bus because that is what they need to do. Truth, Justice and the American Way be damned in the face of political expediency. I don't like that one bit, but I think that is how it will pan out.