Oh, now, after Martin got caught at school with the apparently stolen jewelry and the pry bar, he said they didn't belong to him. No reason to think that's not true. And Martin didn't have any burglary tools on him when Zimmerman saw him, anyway. Next thing you know people will be claiming Martin could have been scoping out houses so he could come back later -- while, say, the owners were at work and he was at home suspended?
And the severity of the head wound Zimmerman actually suffered doesn't matter much. Like if someone takes a shot at you and misses-- no harm no foul? Zimmerman was fighting back when he was injured. If the fight had gone poorly and he couldn't resist the head slamming, what happens to him then? That is why self-defense can be used in response to a reasonable FEAR of imminent serious injury. At any rate, Zimmerman's story is apparently that Martin also tried to take his gun during the fight, which really cements his self-defense claim.
I see two potential problems for Zimmerman's defense. The first is that Zimmerman is somehow cast as the aggressor, in the technical legal sense, notwithstanding that pretty much all the evidence and circumstances support his story. That changes the rules for self-defense. The other scenario is that he's convicted on the basis of people (and at least two experts apparently) claiming that Martin was the one screaming for help at the end of the fight. The idea being that Zimmerman had control was no longer in danger, making him guilty of either manslaughter if he was in the heat of passion (almost certainly the case considering his injuries) or murder, if jpeirpont ends up on the jury.
As far as Martin being killed, it's a tragedy when anyone dies unnecessarily. But I think it's a shame that this incident is going to be used to scare people away from good things--concealed carry, self-defense, watching out for your neighbors, not being afraid to walk in your own neighborhood, and encouraging bad things-- the principal one being the all-too-common use of a racial insult, real or perceived, as justification for physically attacking someone.