There's a BBC documentary about it, I think this one:
It's been a long time since I watched it, but I think the inert gas route is very pleasant. He even gets slightly high/happy from it.
Key takeaways:
- there are surprisingly easy ways to kill people humanely.
- many in the US doesn't want to kill prisoners humanely, they want it to hurt and be a punishment, not die in a euphoric high
edit: found it:
https://www.documentarytube.com/videos/how-to-kill-a-human-being-2/
Rendered unconcious within 15 seconds, dead within a minute.
In testing pigs would happily stick their heads in a space with pure nitrogen and munch on apples till they lost consciousness, fell over, then stick their heads back in the space with nitrogen to eat some more apples.
Read the article, not just the title.