There's not really any way to argue for the white watch officer besides maybe the black kid was insulting him, provoking the cop into assaulting the teen, then shooting him.
The kid wasn't robbing a store, he wasn't buying drugs, he wasn't walking around with a gun out, he did not confront the cop, the cop was following him because he looked "suspicious". Maybe the kid started the fight that broke out, they started wrestling and the kid was shouting for help before Zimmerman shot him.