For the OP, I recall that Zimmerman was advised to get a gun by the police after he complained about a vicious dog that had threatened his wife. I also recall that he was not on neighborhood watch duty the might he had the incident. He was driving out and saw Trayvon and thought he looked suspicious. This is from memory so I may have details wrong.
If Zimmerman wasn't on watch duty, then who was, and how did that individual not get involved? Zimmerman was on watch duty. He left his vehicle, in violation of neighborhood watch protocol. He was armed, in violation of neighborhood watch protocol. The cops let a vigilante with a criminal past carry a gun and we have a not unexpected outcome.