Great topic.
First off, I have euthanised snakes. A neighbor of mine bludgeoned a garter snake with a shovel a couple of months ago just seconds before I realized what he was doing. She was very clearly mortally wounded and in a great deal of pain. After I decided there wasn't anything anyone could do for her I ended it the quickest way I could think of. I gave her a quick shot to the head with a hammer. It was better than watching her writhe in agony.
Secondly, I would euthanise a snake from my own collection if the animal was certified by a Vet. to have an incurable, contagious disease for the protection of the rest of my collection and to prevent suffering. I also think severe deformation is an appropriate reason to perform euth..
As far as "how to go about it....," I think a sharp blow to the head is likely to be the least painful. I can't get my head around freezing a snake....that seems like it would take a little to long and I don't see how it could be painless. I think it is less painful for the owner of the snake because she/he doesn't have to watch or see any blood.
Take care, Chris O
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www.chrisolsonreptiles.com