In 2005, Zimmerman was charged with assaulting a police officer and resisting arrest. The charges were reduced, then dropped when Zimmerman entered a pre-trial diversion program. Also in 2005, Zimmerman's ex-fiance filed a restraining order against him, alleging domestic violence.
Zimmerman's application to be a police officer had been rejected by a Virginia police department. Zimmerman wanted to do police ride alongs. He also chose to participate in the Neighborhood Watch Program. And Zimmerman choose to arm himself, despite the fact that the Watch Program discouraged its participants from doing so.
But at the time of the attack, Zimmerman wasn't on watch. He chose to follow Martin. He chose to disregard police instructions. He chose to get out of the car. As the one "trained" and armed with deadly force, it was his responsibility to see that the situation didn't escalate out of control. It did.
Personally, I'd characterize Zimmerman as a police officer wannabe. He wanted to be a cop and was rejected. So he armed himself and continually went out on "patrol", looking for trouble and a chance to be a hero.
That night he found it. And he allowed the situation to escalate totally out of control. Faced with an actual confrontation, he panicked and resorted to using deadly force. Result? One dead kid.
Zimmerman carries complete and total responsibility for the shooting.