The thing is by the letter of the law I don't think the jury could find him guilty, the fact that there were no clear witnesses to events means it is impossible that there is isn't going to be "reasonable doubt" . Even if you think Zimmerman's story of being punched over a dozen times and having his head repeatedly smashed off concrete is a pack of BS, which going on Zimmer's minimal injuries would appear to be. There is always going to be some doubt as to what actually happened due to no one witnessing it bar the defendant.
But it still doesn't take away the injustice of the whole thing, an unarmed teenager who has just popped to the shop to buy some sweets is pursued and shot dead 80 yards away from where he is staying by an armed man who was advised by 911 operator not to follow. And the law allows the killer to walk away scott free.
I think it's obvious people are going to frustrated by a system that allows that kind of thing to happen.