Good riddance.
Absolutely. Painless, no doctors needed, no exotic drugs needed. Can hardly be called "cruel and unusual," either, since there are occasional industrial accidents involving nitrogen asphyxiation. The victims of those accidents don't even know they're dying.
An accident like that happened to a professional colleague. Just as things started to go black, he realized what was going on, held his breath, and ran out of the room. He survived.