Zimmerman got out of his car, and from a distance ran after Trayvon Martin. That much you can hear on the recorded 911 call. He said he did not go out to pursue Martin, but to see where Martin went and read the street name on the sign to tell police where to go.
After that, Zimmerman said he was walking back toward his truck when Trayvon came out of the darkness.
(I've seen several reporters on tv re-trace the neighborhood at night, and whether "from behind bushes" or just hiding in the darkness, it would be very easy to ambush someone the way Martin did Zimmerman.)


And in plain words without legal definitions, I think if a guy is on the ground getting his face pounded (as a witness testified, Martin was on top, pounding away) he has a right to defend himself.

If Martin had hit Zimmerman once or several times and then walked away, I'd say Zimmerman was in the wrong, because the danger would have been over.
If Zimmerman shot Martin from 5 or 10 feet away (and not 1 to 18 inches, as the autopsy measured from the powder burns), then I'd say Zimmerman was wrong, and the shooting was unnecessary.
But that was not the case.

Witnesses and evidence showed at the time Zimmerman fired, he was being pounded relentlessly. Zimmerman's injuries prove that. And Martin's autopsy showed no injury to him, not even a single punch. By evidence, all provocation was by Martin.
Zimmerman did not initiate the fight, and when pounded relentlessly with no end in sight, had a right to defend himself.