It seems very unlikely that Zimmerman just got out of his car thinking, "I'm gonna kill this kid."
And that's the difference. I'm not so sure.
There are lots of cop/hero wannabe's like Zimmerman out there with chips on their shoulders and itchy trigger fingers. They are just waiting for a chance to "save the world". I think he was a ticking time bomb, and Trayvon happened to be in the wrong place at the wrong time. If it wasn't Trayvon, then six months or a year from now, it would have been someone else.