I just thought of something.
In the past few years, we have heard of botched lethal injection executions, and resulting legal battles over the cocktails, their usage, and of course the 8th A gets thrown into the mix.
On the one hand, the protocols and dugs used are causing and/or will cause the sentenced to suffer. On the other hand, they do not. Then we have 'cruel and unusual punishment'. It is. Until it isn't.
Why not use carbon monoxide as the "cocktail"? Isn't CO completely painless? You sedate the sentenced, and fill the room with CO. Obviously, you get everyone out first, but wouldn't the person just fall asleep, and then expire? Why not do that? Dare I think the reason is that we as a society secretly want someone who has committed capital murder to suffer?