Martin became spooked by Zimmerman following him (not knowing in what capacity Zimmerman was following him with), and took off running. Zimmerman followed Martin and eventually caught up, where the two had a physical altercation that ended with Zimmerman shooting Martin and Martin died from the gun shot wound.
There is no self-defense or stand-your-ground protection from the law, when you run after someone in order to engage them in a physical confrontation.
The KID could have whipped out a gun and shot the guy chasing him, and had a better defense legally, than the fat guy.
The fat guy, while he did a great service for the last decade patrolling the streets, screwed up here. He SHOULD have stayed in the truck, told the police where this kid was. They would have shook his ass down, no doubt about it. But it appears he wanted to be a hero, or thought he was a cop, thought he had the right to chase someone down. he did not. He could have simply ASKED the kid what he was doing there, instead of chasing him. Maybe he could have given the kid directions to the house he was looking for.
The guy worked hard and gave a shit about his community - all true and commendable. But on this night, he left the law in his truck, and let his emotions guide him. He broke the law just by chasing the kid. He broke the law shooting the kid. For someone who cares about the law so much, he sure ignored it once his emotions took over.
