CO2 poisoining is exactly like falling asleep. Which is why you need an alarm in your home to detect it. It has no smell and brings about drowzyness and ultimately the eternal sleep.

It has been decided that is humane because its effects are proven in animals as well as humans. Anyone that wants to whack a rodent against a hard surface can by all means make that choice, but the fact that people don't see an issue with it is baffling.