What probably happened.
Zimmerman saw another "****** punk" who always gets away walking with arrogance (in his mind) down the street in a "white" neighborhood.
So he started following the kid imagining every thing his psychologist was afraid he would imagine. Then, when the kid ran off, to Zimmerman, that was "proof" and got out of his car to chase down the imagined perpetrator.
At this point, Trayvon is walking home with his Skittles and Ice Tea with a bounce in his step knowing he would be watching the "game" he had been waiting for. Then he notices a big ugly older white (to him) guy following him. He wonders if it's a perv or a policeman. Since the guy doesn't call out or yell stop, he thinks it's a perv so he takes off running.
Thinking he's safe, he begins talking with his "friend" girl telling her about the perv who was following him when he hears Zimmerman saying something like, "Where do you think you're going ******?" So Trayvon tells him, "Stay away from me" and starts to walk off. At this point, Zimmerman pulls out his gun and takes off the safety. He grabs Trayvon and says, "You punks always get away. Well not this time". Because he was so close, Trayvon grabs the gun and jumps on Zimmerman and they both go down with Trayvon screaming for help.
They fall back and Zimmerman hits his head on the sidewalk with the uneven surface which is why he has only a couple of slight scrapes. That's when Zimmerman shoots the kid who falls forward and his forehead hits Zimmerman in the mouth. Trayvon doesn't show bruising because he is already dead. Zimmerman pushes Trayvon off to the side which is why they found Trayvon face down with his hands under his chest.
Zimmerman was able to shoot so quickly because he had already disengaged the safety. He wasn't letting "this one" get away. Too many others already had.
By the time the police showed up, Zimmerman was walking around the body like a game hunter proud of what he bagged. He didn't feel remorse, but he didn't want to boast either. After leaving the police station, he went out and celebrated.
I suspect that was pretty close to what actually happened.
Right wingers are hoping this incident gives them license to hunt down any black man or child they want and have no fear of repercussion. They feel they are teaching "uppity" blacks to not get above their station. In their minds, they are actually hurting Obama somehow.