Snakes, reptiles in general are similar to fish - at least in my mind. Fish and reptiles are ectotherms. The video of the detached rattlesnake head on page one demonstrates what I have seen with sharks that have had their head removed. They still continue to "live" / move for several minutes. I wouldn't want to see that as I believe the animal suffers.
I would talk to my vet and ask for an injectable prescription to euthanize it at home. If he would have seen the animal previously/recently to treat it for what was ailing it, I might have a chance at getting the prescription without a vet visit, and just pay the cost of meds.