I think carbon monoxide would be the least painful.
The build up of Carbon Monoxide triggers the feeling of suffocation, since air is 70% Nitrogen the body doesn’t recognize it as foreign, so you simply fall asleep.
Nope. Nitrogen is. It’s inert, build up of CO2 is what causes the body to want to take a breath. With no O2 no CO2. You just breathe normally until you pass out and die.
I would chipper would be more or less painless. Not to mention more satisfying for the victims families.
Ask anyone who has almost died from carbon monoxide poisoning - it’s horrible - worst headache ever, severe nausea, throwing up - least painful?
Not so.
Been there, done that.........
Carbon Monoxide is not, from what I have read, an easy death. Nitrogen, OTOH, is a definitely inert gas that is naturally in the air we breathe, thus the body does not react to the suffocation the same was as other gases.