I worked for a company where several employees died in the 70’s from a leaking nitrogen line. Nitrogen displaced the air and they all died at their stations apparently unaware there was a problem. There’s a suicide kit for sale that includes a plastic bag and a small canister of (Helium, I think.) It promises you’ll feel no pain and no discomfort. (I have to wonder if there’s a phone number on the package for complaints.)
In any case, it sounds like this approach would do away with 90% of the problems with executions.
So would old fashioned gallows.
Nitrogen execution would also allow the condemned to donate his / her organs if they chose. Even hanging and the guillotine would allow that. Why we use incredibly toxic chemicals is confusing to me.