Based on the repeated and excruciating screams heard a minute before the shot is fired, http://soundclick.com/share.cfm?id=11548279  Zimmerman had the opportunity not to shoot Martin. Since he shot him anyway, the prosecutor had enough evidence to bring a murder charge -- this is so even if we assume that Zimmerman was initially punched and knocked down.  Martin’s “help, help” pleas show that Zimmerman later got the upper hand, and could have waited for the police to make the arrest.  Instead, he shot an unarmed man pleading for help. This is enough to show criminal intent and a murder charge was therefore in order.
As for race, it had nothing to do with Zimmerman’s decision to shoot -- though it may have been a factor in his pursuing Martin at the outset.  It certainly was a factor in the perfunctory investigation that followed.  Nonetheless, Zimmerman did not shoot Martin because he was black. He shot him because he was angry.  So angry, in fact, that he could not hear Martin’s desperate pleas for help.
Great anger, even when provoked, does not justify the killing of another human being.