Who decided that causing asphyxiation with CO2 is the most humane? For all we know it could be more painful and terrible for the mice than a quick thump on the head. How do we know they just "slowly fall asleep" from breathing CO2? I can't imagine suffocating would be any more pleasant than getting knocked instantly unconscious.

If someone wants to prekill by thumping the rodent then it's their choice. There are many animal rights activists who would say everyone on this forum treats animals inhumanely by keeping them in captivity in tiny enclosures. No matter how humane you think you are there will always be other people who think you're doing things wrong.