The issue is Zimmerman should never have gotten out of his car. He didn't catch Martin in the act of doing anything wrong, so he should have left him alone or watch from his car at most.
Yeah, there's a saying that covers that...
Hindsight is 20/20
No, it's not hindsight when it's something you should know ahead of time. Zimmerman was doing neighborhood watch, had taken law classes, and wanted to be a cop. He should have known that mildly suspicious behavior barely even warranted him to call the cops.