They should go with nitrogen smothering. It’s quick and the scumbag won’t even know he’s dying.
Agree!
Likely even better then CO.
Nitrogen
I have worked in some refineries where some workers died of asphyxiation. An example is where a confined space is flooded with nitrogen. A worker just opened the hatch, did not do a job hazard analysis, no hole watch, and just climbs in. He never knew what hit him. He passed out, died of anoxia. Someone else impulsively climb in to rescue him, and he went down.
With nitrogen, unlike hydrogen cyanide or carbon monoxide, has no discomfort. It just shuts the body down.