No matter how good intentioned he was, Zimmerman was the aggressor in this instance. Martin was walking along the pathway (maybe running because of the rain) chatting with his girlfriend, going to the apartment.
If Zimmerman had waited for the police by his truck or by the mailboxes, this incident would have never happened. Who shoved first, who punched first, who knows ... does it matter? Zimmerman panicked and the kid's dead.