It was not a “horrible death” if he was on top of Z, bashing his skull against the ground. People get killed that way. “Stand your ground” is moot if you are on your back getting beaten into unconsciousness or death.
I've probably missed it, but I haven't seen a lot of discussion about why Zimmerman's lawyer seemed to take Stand Your Ground off the table. At first, that had a bunch of people scratching their head since it seemed like an important and beneficial statute -- but the lawyer indicated that they would not use it.
Of course, as you say, no one needs Stand Your Ground if they are flat on their back, getting their head smashed. In that situation, the shoot can easily be seen as righteous by any rational human.