Zimmerman had a huge opportunity to drive away from a potentially dangerous situation, if he was telling the truth about Martin circling his vehicle while he was in it. It's called turn the key, put it D.
But he didn't, did he? No....he got out and followed the kid...why would he do that? Oh yeah....he was armed....sense of security....need to be the hero....whatever. That very act alone could be his undoing. A situation that he could have completely avoided.
That's what you call "wanna-be" cop.