Nitrogen Asphyxiation:
strap the condemned down
attach a brain wave monitor
put on a mask
supply 99% pure nitrogen (cheep welding gas) at 1.1 atmospheres
condemned passes out
wait till brain waves go flat
THEN...
electocute em, hang em, shoot em, guillotine em, then defenistrate em.
One of those ought to stick.
The beauty of nitrogen (or helium) asphyxiation is that it prevents CO2 buildup in the blood. CO2 buildup is what triggers the urge to breathe. So you pass out without feeling suffocated.
However, it isn’t sustainable because the gas is exhausted by the guest of honor and has to be replaced. A Guillotine is reusable and only needs a bit of maintenance after each use (=sustainable).