The question is more if you HAVE to kill them. Lets say the snake has proven to not be a strong eater after a month and has a kink putting it's neck at a 90 degree angle. You would have to do something instead of just letting it starve to death.

I suppose dry ice in water would also cool the reptile down making the death take longer (as it would breathe less) so that would be bad. But what about decapitation? Messy business but no blood to the brain means fairly quick death, a mallet to the skull would be faster still but more likely to mess up (not hitting hard enough).

Its sad business in any case and Ralph Davis mentioned in a video that he puts them in the freezer.