There was a case in WA, where the slimebag gained weight so he couldn't be hanged, since WA still had that. It was found to be cruel and unusual punishment. It's not like they aren't sitting on death row for a reason. Firing squad works.
I wsa thinking at the beginning of this article that if he';s too fat to drug up, all that extra weight it would probably increase his chances of a very quick hanging death- much easier to snap the neck with all that momentum.
Also, as others have said, bigger target for firing squads. Hey, "increase chances" for one, increase chances for another. He can pick his death from all of the above, as far as I'm concerned.