The only job of a neighborhood watch person is to call the police, not to go after the bad guy themselves.
It sounds like Zimmerman did just call the police, and possibly went back to his car and was attacked by Martin, or possibly continued to follow Martin and was attacked by him. Regardless, Zimmerman was obviously defending himself...but he shouldn’t have been armed in the first place if he was acting in his capacity as neighborhood watchman.
The police were on their way so he wouldn’t have been any more seriously injured than he was, none of this would have happened, and Martin would probably now be sitting in a jail cell for assault and battery on a designated neighborhood watchman.
Why not? Is this America, or Nazi Germany?