Quote from MarketMasher:
Strictly as a hypothesis....
1) There have been burglaries in the area.
2) Zimmerman spots Martin. He does not recognize him, so he starts to follow him. Perhaps he calls a dispatcher, who then informs him not to follow his suspect.
3) Martin notices that he is being followed. He calls his girlfriend. He picks up his pace.
At this point, perhaps Martin does the obvious thing, like turn down a one-way street where Zimmerman cannot follow in his car. Something like that would force Zimmerman out of his vehicle to continue on foot. This would be a conscious choice to pursue.
4) Martin turns and confronts Zimmerman.
5) Words are exchanged - it becomes a pissing match.
6) Martin is younger, though hard to say if one of them or both of them lack discipline. Whether on-the-spot, or Martin follows Zimmerman back toward his car, a physical altercation begins. At this point, it may be Martin who is the aggressor, fueled by anger.
7) Outmatched physically, Zimmerman pulls his pistol and shoots Martin, killing him.
If Martin assaulted Zimmerman, he was clearly in the wrong. However, it was Zimmerman's choice to deal with the situation himself rather than wait for the police to arrive. Zimmerman is not a law enforcement officer and apparently has no training as one. He disregarded instructions to not follow/confront Martin.
So now the courts get to decide.