I think there are times when someone feels threatened and therefore shoots in a type of self defense. However, everything that has come out about George Zimmerman says that he is an angry man who was out on the prowl for kids that fit a particular racial and other profile, such as how they were dressed. He did not shoot in self defense.
This is a very volatile issue, and as well it should be. Any use of excessive force should be treated illegally, and in cases where the victim was fatal, should be treated as a federal offense. Trayvon Martin was a minor; under 18, and was unarmed. Zimmerman was an adult, and even if he was met with violence, he shouldn't have fatally wounded Martin. If anything, a shot to the leg or any area below the waist would've been sufficient enough force to stop the altercation.