I couldn't have said it better. You can gas properly or break necks properly. We are all arguing the polar ends of each side. What has to be heard though is there is proper ways to do both. People should not get flack for choosing their method.
If you truly care about animal welfare you would raise your own animals or know the farmer who raised them. If you have ever bought food from a grocery store you can never ever discuss animal ethics to me. Ever. Eat anywhere but your house? That chicken cow pig or turkey was raised in a 1x1 or 3x3 it's whole life. Forced food and had blisters all over it. Do you drink store bought milk? The blood and puss from those poor cows udders is bleached out and homogenized. Because those poor babies are hooked up to vacuum pumps that milk them dry to the point of death. I raise and grow all my own food. I raise organic mice and rats. Yes. My rats and mice eat better than most humans. Every one of my animals is raised and slaughtered humanely. Take care of something ever day for a year then kill it. Tell me you wouldn't want it to be done as fast and with as least pain as possible. If it was acceptable to gas animals we would have gas houses for animal production. Not slaughter houses. Be real. Think. If we are getting into an ethics discussion rather than a question about how to properly kill mice without co2 then let's do that dance. But.
OP wanted the proper way to kill mice without c02. He asked me in PM because he didn't want to get ridiculed more. Grow up people.