The racial component should not be the issue, but it is. It's the driving issue in this case. If the same incident inculded two blacks or two whites or the situation reversed, none of us would have ever heard about it. It would have remained local and, with the information the police say they have, no one would have been arrested.
That being said, it would have been wise for Zimmerman to call the police and WAIT in this case instead of confronting the thug on his own. Now if he was actively assaulting another person or seen crawling through a window...then yes. If the situation is not HOT....WAIT FOR BACK-UP!
But does not waiting for back-up make what he did a crime? No