* Our atmosphere is around 80% nitrogen so usage can't be restricted, very inexpensive to purify, doesn't consume resources needed elsewhere (ie medically)
* Painless and humane: the victim just goes to sleep. They may become giddy beforehand
* No risk of leaks or poisoning as long as the areas around the chamber are open to the outside air... the chamber needs only be moderately airtight
Ideally this would be the time to reflect that perhaps, after numerous proven instances where innocent people were put to death or narrowly avoided it with a death-row exoneration, that a 21st century civilized society should abandon this barbaric practice, but if saner heads don't prevail at least there is this ideal method of it.