If you can find it https://www.imdb.com/title/tt1228865 is a good watch. It covers other methods, but it becomes clear how fast, effective, and painless nitrogen asphyxiation can be, as the presenter has to receive assistance in order not to die while attempting to get close to the experience (without dying).
It's also a bit sad, as it makes it clear that for at least some capital punishment advocates, suffering is a desired part of the outcome.
I'd like to avoid death, but I can foresee a potential future when my quality of life is negative and no amount of volunteer effort can bring it positive. If that happens, I'd like to opt-in to mortality via inert gas (probably nitrogen) asphyxiation.
If we must have capital punishment, inert gas asphyxiation seems to be the best known way to do it. I'm not convinced we must have capital punishment, tho.