Ok, but bear with me here, because for real, this is how I want to go, and how I plan to put down my fowl when they get too old to live comfortably, because there’s no stress involved to taint the meat, and I can feel comfortable with myself for giving them a good life with free roam, and a good end.
It’s incredibly humane. You feel nothing and don’t know you are suffocating. If you’ve ever breathed helium, you know what nitrogen feels like - literally nothing. This happened to multiple individuals in space because nitrogen is not flammable, and is why they now use 6% co2 in non-oxygenated spaces.
The body does not care if it has oxygen, that’s hard to test for biologically because oxygen is highly reactive, what it does test for is buildup of co2. As long as you can breathe out the co2, your body knows nothing.
So if they are going to kill other humans, this is the way to go. I don’t agree with doing that non-voluntarily, but if it’s going to happen this is at least humane.