Snakes can hold their breath for hours, which makes carbon dioxide from dry ice and drowning no good for euthanasia.
The veterinarians recommend a dose of anaesthetic injected into a snake's body as most effective. They believe that the formation of ice crystals in the snake's body during freezing would be very painfull. If no anaesthetic is available, then they believe that crushing the brain with a hammer or similar tool is least painful.
I've killed snakes that I found badly injured on the road.
Holding the head still enough to hit is difficult. IMHO, putting the snake in a 40 degree Fahrenheit refrigerator overnight should cool it to a comatose state where the head will stay reasonably still and may be enough to kill the snake without forming ice crystals.
Paul Hollander