I don't think GZ has said he was being beaten so badly that his only choice to save his life was to shoot. I think he said he feared for his life.
I'm just going to ask this, honestly. if you had a gun and someone you didn't know was on top of you, how long would you wait to use it? I'm not trying to pick a fight, I'm just asking you to imagine yourself in GZ's position, with some documented (maybe minimal injuries to his face and head) on his back on the ground with Trayvon on top of you, how long would you wait to fire?