[Tserisa]

Frozen mice from commercial operations are almost all killed through CO2, which is considered one of the most humane ways to put an animal to sleep, while preserving the safety of feeding the animal to another. It is certainly more humane than hitting them, or being struck and suffocated by a snake. It is also cheap for large numbers of animals, so even feeder breeders who are less than compassionate about their rodents tend to use it anyway.

As for how they were treated before death -- that's where it usually gets particularly awful. Feeder rodents in commercial organizations are rarely treated well, kept in racks with no stimulation, etc.