I know that when I was seventeen, if I was walking through neighborhood and it was dark, cold and rainy and somebody started trailing me in an unmarked vehicle and then got out and approached with even a hint of aggression it would have been on. Most people would be pretty defensive in that position. The tricky part is that, with regards to the law, the role of "victim" can change quickly based on circumstances.
The "Stand your ground" law looks like it's going to be in Zimmerman's favor for the simple fact that even though he forced what I would consider a completely unjustified confrontation he is arguing that he was getting his ass handed to him in the physical altercation and then suddenly his life was in danger. If you look at the bottom of this page (in the link below) there are some historical examples of the same law getting people out of some pretty similar scenarios
http://www.tampabay.com/stand-your-ground-law/cases/case_139.
In my personal opinion Georgie boy fucked up big time and, unfortunately, there are no take backs when you shoot somebody. Fuck him.