There are mutiple herp vet negative responses to Freezing as a humane method of euthanization (they can be found on the green iguana sites):
1. The refridgerator makes the reptile torpid, unable to move, but they are not asleep, when put in the freezer.
2. The external dermis and eyes freeze first forming ice crystals in the cells. In mammals this is a painful process, the reptile however, when put in the freezer cannot move or react to the pain because, being cold blooded, they have been rendered relatively immobile by being in the refridgerator. The brain and heart freeze slower, leaving a living animal to develop ice crystals through its body.
3. This used to be the preferred method for euthanizing reptiles, CO2 is the preferred method, per my herp vet, but in a reptile this takes a bit of time, as they can hold their breath and as their breathing slows down, so does their CO2 intake.
4. Lethal injection is also slow due to the fact that unless the snake is injected in the heart, the medicine slows down the heart so rapidly that it does not circulate the entire dose quickly.
I did extensive research on this issue during years of working with iguanas. Many having come to me VERY disabled and beyond saving.
I use injectible valium from the vet, subcutanious, and when sedated, ensure that I have a clean hit on the brain (not a tap- but flatten the whole head)
I would love to believe that they just fell asleep in the fridge and freezer and that would be so simple and kind, but its not the way it works and I am responsible for ending their lives as humanely as possible.