Maybe he did, maybe he didn't. We still don't know the full story. It's highly unlikely he walked up to Martin and just shot him. Did he say things to Martin? We don't know. I know if I'm a NWV like Zimmerman, and it happened like he said it did, Martin attacked him from behind and threatened to kill him, then he acted like anyone else would act if someone was beating their head against concrete. But let's wait and see if more evidence emerges.
I agree that we do not have all the facts and I do not think that he should have been charges....but we do know that this would not have happened at all if he had not gottenbin over his head. An untrained dope with a gun who follows a “suspiscous” person is just asking for trouble...and regardless of how it started zimmerman found it.