Inert Gas Asphyxiation seems the way to me.
NITROGEN - Cheap, available, relatively harmless to the masses, no nasty chemicals to clean up, much easier to administer.
The process seems to be 'proven' by all those that have taken the 'forever nap' from being in an enclosed space with the engine running. Doesn't seem there was a 'struggle' or pain involved.
Europe uses CO2 and Helium, to put pigs down.
https://www.youtube.com/watch?v=cqaPZR4M22w
