i guess it's never occurred to me to ask how PET STORES kill their mice
it seems very likely that they just toss the mice in the freezer. but no matter how the mice get frozen, buying frozen mice from a pet store is no more humane than freezing them yourself.
and no matter how it's killed, the mouse is going to go through a good amount of pain when it dies. plus the confusion and fear that it experiences if you feed it live.
owning a snake involves accepting that they eat mice, which involves accepting that at least one mouse will die at least once every week or two by your own hand (even if you bought it dead, you're contributing), which involves accepting that the mouse will feel some sort of pain.