Vegan

I believe, there’s a way to gas pigs that just makes them sleepy and then pass out. I think, it was by using nitrogen. But yeah, that’s not commonly done.

IIRC, it's literally any other gas except CO2 - the human body detects suffocation not by the lack of oxygen in blood, but the presence of carbonic acid (which is acidic). Since every other gas doesn't create carbonic acid (except maybe some gases that contain carbon?), they all silently/painlessly kill.

Nitrogen, argon, or hydrogen would all work and be fairly cheap. Hydrogen would be explosively flammable though, so you could combine two steps into one!