I’ve heard of using the freezer method for reptiles, but not for mammals. Supposedly, since reptiles are cold-blooded, they just nod off and go to sleep. But I’ve also been told that this is inhumane because they’re still alive when the ice crystals begin to form in their cells, and when the cell walls rupture it causes a painful death, so I’ve never employed this technique.
I think Honey is on the right track. A few years ago we had a female hamster that we were convinced was the reincarnation of Adolph Hitler. Mean little shit. Bit me so deep once that my finger bled for two hours. Escaped from her cage and built herself a bunker in the entryway broom closet. It took us six months to catch her. The little bastard survived by eating the drywall. Anyway, one day she started developing huge tumors. Some of them were almost the size of a golf ball, and had tumors growing on top of the tumors. My wife and I tried to off her by feeding her peanut butter laced with phenobarb. We fed her enough to kill a Great Dane, and she just lapped it up like cotton candy and looked at us for more. The vet wanted $45 to punch her ticket for us. We just stuck her back in her cage, which by then had been fortified with motion-sensing Laser beams and Concertina wire. She continued to thrive, eating like a horse and spinning away on her little wheel without a care in the world. A few days later we found her mortuus on the bottom of her cage. When we buried her in the backyard, nothing ever grew over that spot.