What happened is obvious:
- Zimmerman, in his car, sees Martin "acting suspiciously" (walking while black, ducking into a doorway during a rain shower.)
- He calls 911. The operator tells him *not* to follow Martin and that police are on the way.
- He follows Martin anyway. He is carrying a gun in violation of his "neighborhood watch" policy.
- Martin freaks out.
- At some point, Zimmerman stops his car, gets out, and confronts Martin.
- Fisticuffs ensue. Zimmerman, despite outweighing Martin by 100 lbs, gets the worst if it.
- When Martin has him on the ground and is slamming his head into the pavement, Zimmerman assumes he's in deadly danger, pulls his gun, and shoots Martin.
My own cynical take -- white man shoots black teenager == justified. This is Florida, after all.
"I know how good a gun feels. It makes you bright-eyed and bushy- tailed, three meters tall and covered with hair. You're ready for anything and kind of hoping you'll find it. Which is exactly what is dangerous about it-because you aren't anything of the sort." -- Tunnel in the Sky, Robert Heinlein
Or, as somebody else put it more pithily: "Guns make you stupid."