Just to play devil's advocate, it could be that Zimmerman confronted Martin verbally, and Martin initiated the physical assault, which may have justified the use of deadly force by Zimmerman.
I'm rather skeptical of that theory myself, but it could have happened. We don't have all the details. Nor do we know how active the investigation was before this hit the news.
My impression of Zimmerman, just from reading the news, makes me think he's an individual who would have physically tried to stop Martin, if Martin tried to leave. I also wonder if this case wasn't being strongly investigated before it hit the news. (Oh look a dead black teen in a hoodie, who cares?)
Personally, I like "stand your ground laws". I base this on my own habits - I occasionally have to travel at night through sketchy areas. If I'm ever assaulted, I may have a hard time fleeing from someone who has a willingness to use deadly force. The reason is simple - do I really want to turn my back on someone with a weapon? No. But that being said, I don't think this is one of those situations, nor is it something the law was intended to allow. People have a right to defend themselves in a bad situation only if they didn't cause the confrontation in the first place. I suspect Zimmerman either physically restrained or otherwise used force to stop Martin, and in that situation, Zimmerman is responsible for what happened. If the facts of the case shows that, Zimmerman should be found guilty of murder.