The fact that the mob has no legal grounds to exact their revenge upon Zimmerman only means that they'll resort to taking 'justice' into their own hands.
GZ will be a marked man for the rest of his days. He may not be going to a concrete prison, but his life from this point forward will be a prison itself. His only hope for real freedom, is to one day slip quietly out of the country and relocate to a place where no one knows him.
I'm not even sure that's possible, given today's interconnected world.
If I were GZ, I wouldn't be too overly concerned about that, since coming future events will likely change the public's perception at large.