But the head bashing isn't the core aspect of the "self-defense" claim . . . That is a product of the idiot media.
Zimmerman's claim of self defense arises when, as Martin was straddling him, punching him, slamming his head into the concrete, Zimmerman's body was on the grass and only his head was on the concrete. Zimmerman thought to begin squirming on the grass, down the sloping ground to get his head off the concrete and in doing this, with Martin on top of him, his jacket slid up, exposing his holstered gun. Martin saw the gun, told Zimmerman, "you are going to die tonight Motherfucker!" and the altercation turned into a gun grab situation.
In a gun grab all the other considerations go out the window, an "unarmed" assaulter can become armed in a millisecond and resisted with lethal force.
None of the 911 transcripts and BS narratives that flow from those tapes matter at that point.
There was no evidence that night nor any presented since, that can refute Zimmerman's account of the altercation which is why Zimmerman was not charged with any crime (until the racist / political shitshow started).