Personally I think suffocating your rodents through CO2 is actually fairly cruel.

Whether or not something is considered "humane" is entirely based on "does it look gross or not?" and has nothing at all to do with what truly creates the least amount of harm. Cutting somethings head off isn't considered humane, however it is the absolute best way of killing something since it is instantaneous and painless. However shoving an animal in a box that is flooded with Carbon Dioxide where they will slowly die of suffocation, unable to breed and in distress.

Not saying I wouldn't do that, or that people shouldn't or that I wouldn't purchase animals prekilled in that way. But the fact that it's considered humane while severing the spine with an axe blade isn't makes one re-think the definition of humane. It's like how injecting a human with poison or sticking them in a room full of Chlorine gas are both considered humane ways of killing them, and the guillotine and hanging are not.