Like you said, you have no evidence regarding who attacked who first.
As for the bloody nose, one blow can cause a person to lose consciousness and then be completely defenseless against further mayhem. It happens all of the time. Ghetto thugs knock someone out and then proceed to maim or kill them. There is no reason for Zimmerman to risk this. This whole "it was just a fistfight" line is garbage. If you attack an innocent person then you deserve what you get. I do not know whether Zimmerman was completely innocent, but I have seen no evidence indicating otherwise.
Many on this thread are assuming, because the dead kid was black, that he was a ghetto thug.
I disagree with this assessment. But the boy wasn’t after Z to stop and explain his presence, but Zimmerman was after the kid to stop and explain his presence, so I still think that Z brought a gun to the fist fight.
It’s just my opinion, but I’m sure this warrants further examination by the authorities.