I think he was a hot head and probably should have minded his own business. And I think it's within the realm of possibility that, say, Zimmerman approached with his gun drawn, Trayvon retaliated and got the jump on him, and so on and so forth, leading to a manslaughter. There's just no evidence.
I don't think he's culpable, morally or otherwise, for watching out for his neighborhood, even to the extent of following some kid down the street. And I don't think he's culpable for carrying a gun. If things happened more or less the way he said they did, then I don't think he's culpable at all. Maybe he should have tried talking to Trayvon, asking him what he was doing, but that is just a mistake. That doesn't make him a bad person.