I think an important point is that in his written statement, it wasn't the 'head bashing' which prompted Zimmerman to shoot Martin...it was that Martin "assured" Zimmerman that he "was going to die tonight." In other words, and according to his own statement, Zimmerman's concern/thought/belief wasn't that he could/would die from the "head bashing".
(Re: Zimmerman's statements, I want to stress that I'm highlighting what HE has said happened, not what I personally think or believe happened.)
In both cases (if true), his life was threatened. He had a right to defend himself. It was six of one and half a dozen of the other. I wasn't there, so my comments were speculation just like the rest of the posters in this thread.