As gross as it is to do, I'd say that smashing the head with a heavy object is probably the kindest way to kill a snake. The brain integrety is immediately destroyed and there is no way for pain messages to be perceived.

I've seen the infamous YouTube video of a bunch of morons who cut the head off of a snake and kept tormenting it while the head kept trying to defend itself. Sickening. The brain was intact and still able to perceive a need to defend itself so that animal could no doubt feel pain, fear, oxygen deprivation and helplessness when it was unable to flee. I can't even begin to put into non obscene language how I feel about people who think it's fun and exciting to watch some poor animal suffer and die a slow horrible death. They are a waste of the air they suck in when they breathe.