At this point, I think the only possible solution, aside from the obvious and cheaper life without parole, is to move to a pneumatically operated guillotine. We know that any post-separation convulsions are entirely disconnected from the brain. Sure, it’s a little messy, but retribution and vengeance have their drawbacks. Just clean up the mess or stop executing people.

On a realistic note, I would not be surprised that holding his breath led to his “torture” with CO2 building up in his blood as he intentionally writhed and resulting in actual discomfort as the body reacted to the CO2 even as the lack of oxygen in the breathing mix caused him to lose consciousness. It’s his final act to make a posthumous case (real or sensational) against his executioners. I find it hard to imagine that trace CO2, in even welding N2, would be sufficient to cause a reaction unless they intentionally got a gas mix (I don’t weld with N2, but 75Ar/25CO2 is very common for MIG).