Umm wow no. I doubt that’s legal or humane.
It’s not humane, no. Probably legal, but not humane, freezing causes ice crystals to form in the veins. It’s not a humane way to kill anything, except, possibly, insects, and that only because they may or may not have pain receptors.
If you must kill a reptile, destroy the brain as quickly as possible. A hammer does the trick. It looks nasty, but it’s the most humane method- an animal can’t feel pain if it doesn’t have a brain to feel things with.