Do I think he probably put himself in a position he shouldn't have? Maybe.
But from what I've read about the evidence, it seems pretty clear that at the time he shot him, George was down on the ground with Martin on top of him, and Martin was beating him and probably pounding his head into the ground.
From everything I've read, what I think probably happened: George did follow Trayvon, who got ticked about it and confronted him. There's no way to tell from there who did what that led to the rest, but it ended up with Trayvon on top beating George (at the very least - I think I read that George claimed Trayvon looked like he was reaching for George's gun too, but don't quote me on that!) At that point, George was afraid, had a gun, and used it.
Do I think it was self defense? Yes. Do I think that there was fault on both sides that led up to the whole mess? Yes.
In the end, from what I've read, there's no evidence that George ever actually attacked or physically threatened him - just followed him. And there is evidence that George was being beaten.
I do think it's a tragedy that should never have happened, but that there was likely fault on both sides. But if the only thing George did was follow Trayvon, and Trayvon attacked him, then the thing probably did get to a point where the shooting was self defense. Even if George probably shouldn't have followed Trayvon, that is no excuse for Trayvon attacking George.