Carbon monoxide poisoning.
What I don’t understand about the states and capital punishment is that they all buy in to all of these overly complicated execution systems.
All they need is an airtight chamber and some industrial gas. Perhaps an evacuation system for the gas after the execution is accomplished.
The more complexity they add the more likely is failure. It is an engineering principle that is legendary.
I could also add to the list of painless execution methods: an airtight chamber with a carbon dioxide scrubbing system. The condemned suffocates painlessly as he uses up the available oxygen. The pain of suffocation is caused by the buildup of carbon dioxide in the body and not the lack of oxygen. The condemned will actually feel drunk as death nears.
The system does not even need to be that complicated.
A medical breathing masked hooked up to a bottle of 99% nitrogen (cheep welding gas) and turned on at 1.1 atmospheres and then wait till the brain waves go flat.