Personally, I'd love to see him strangled, with his legs kicking helplessly in front of him, with his tongue protruding and his dick swelling and spurting until all that's left is his lifeless jerking and shuddering...
I would show him several different options, haul up hanging, gas chamber, electrocution, garrotting, impalement, crucifixion. make him watch fellow prisoners die by each of those methods and then I pick the one that it looks like he fears the most for his execution.
have the ones condemned to be whipped made to watch whippings with each of those whips. the one that terrifies them more should be the cruel whip used on them for their whipping.