If Zimmerman was black, called to say that there was a string of burglaries in the neighbourhood recently, described a white teen as looking at all the houses, and as carrying something he couldn't make out, and said "these ***-holes always get away with it", then disobeyed several dispatch suggestions then orders, to wait for police and leave the teen alone, then I'd be screaming for justice for the victim too.
I don't think you would. More correctly, I don't think you'd have ever heard about it because there would be no national news story. No million signatures on a petition demanding action by/against the police. No NAACP folks showing up and demanding justice. No outrage at all. You would never have heard about it.
You do realize that the only reason you know about this case is because the victim was black, right? It fits into a racial context which groups like the NAACP can use, so they swoop in and raise awareness and garner national attention over the issue. You respond to it because of that, not because of the case itself.