If it were a white kid in a hoodie, and a black man following and then shooting the kid we wouldn't be having this discussion. The only reason Zimmerman thought the kid was suspicious was that he was a black kid in a hoodie in a gated community. He was told, by police dispatchers, not to follow the kid, he did anyway. There was an altercation, allegedly, and the unarmed kid was killed. If a black man in an SUV followed a white kid and the white kid ended up shot, we would be calling this a drive-by.
I don't know what happened, and neither does anyone else here. Most of the evidence indicates that Zimmerman followed Martin for no other reason than being black. Evidence also indicates that Zimmerman liked calling 911, 50 times in a year or once every 10 days or so. To most, this would seem that he was unduly focused on others possible misbehaviour. It also looks like Martin had gotten into fairly typical teen trouble; tardiness, graffiti, and possibly pot. There is no indication this kid was violent or aggressive.
The police have fallen down on this. They did not question the multiple 911 callers, they did not drug or alcohol test Zimmerman, they did not follow-up with the girlfriend who was on the phone with Martin, and the simply took Zimmerman's word that it was self-defense.