Pretty sure skiploader had (or posted in?) in a similar thread and actually sited information for why CO2 is not an efficient or humane way to euthanize a snake which contained links with sources. I'd have to go digging for it and I'm at work rn so that may be a bit. Long story short: snakes and other reptiles do not metabolize CO2 the same as mammals, they do not quietly slip into unconsciousness and pass within a few minutes. It's for the same reason that heads remain conscious and able to bite even hours after decapitation. Their brains do not require the same oxygen levels to continue functioning and so the animal does not loose consciousness but is awake and alive for the whole process. Last I checked, pithing where in the brain matter is completely destroyed was still the fastest, most humane way to dispatch a snake, but one that takes some practice to do skillfully and effectively so as to not cause the animal to suffer.