There is a lot of ethics with this question.

CO2 and Freezing are not good methods.

Pithing destroying the brain case is the most efficient way short of a vet.

That said I feel that if you are breeding you should be prepared to euthanize this way it is part of the responsibility of breeding. If you can't bring yourself you should not be breeding.

For the general pet owner I believe the answer is different. Euthanizing a pet is not expected it does however occur. Pithing is the best home way, however botched is worse than anything else. For a pet owner the fastest way that that individual can manage is the best way. If my pet snakes were seriously hurt and needed to be mercy euthanized I hope I would be able to pith and save the agony. My partner is disabled, and tender hearted, there is no way she would be able to pith our pets. I work 45 min at best away from home, not soon enough freezing would be better than suffering longer. In her case freezing is better than nothing.

Mercy euthanizing is about the fastest efficient way to end suffering. A general pet owner I feel at least does not need to be prepared to euthanize a pet. Breeders I feel should be prepared to do this it has a much higher chance of being needed.