That sounds an awful lot like, "She was wearing provocative clothing". Zimmerman may have been in the wrong following the individual. Having been in a similar situation I'd call it over-zealous in protecting his own, to the point of putting ones self in unnecessary danger.
If the story is true that Martin threw the first punch and began assaulting Zimmerman in the manner described (banging his head on the concrete), then Martin is the one that crossed that line, not Zimmerman. Add into the mix that no help came when he called for it (when you need help in second, the police are minutes away), then as far as I am concerned Zimmerman isn't guilty of anything other than protecting himself.
Why? If I was bleeding from nose and had my head bounced around the concrete, I sure would go.
Head bangs to concrete - what if he had concussion or bleeding brain.
He had a broken nose? Again, why wouldn't he go to hospital?