I've gassed snakes before, you can keep them in CO2 for a really long time and after you take them out they can revive themselves , it's amazing. CO2 isn't perfect but if you read state laws about euthanization its the most legal and accepted method for all animals. I used to breed canaries and when I started I froze all my deformed chicks, apparently that's not legal and supposedly a very painful way to go. Another approved method is an explosive device like a gun or bomb LOL. That's probably the quickest and most humane but may be the most dangerous for the user, I wouldn't recommend it!
Must order some tannerite. Just kidding. I think would rehome any undesired snake over killing it. If was not healthy and had to kill it, not sure how I would at this point but would research more before possibly finding myself in that position.