Hmm.... well knowing accidents happen as I am sure you didn't intend to hit it with the weed whacker and tried to put it out of its misery in the quickest way possible. I am surprised as well that the body is still twitching etc. I know that snakes can move around for a fair length of time after having their head severed but 7 hours seems a little extreme.

On a good note, I highly doubt the snake is aware of any pain...as it no longer has a brain to acknowledge nerve signals from the body. It may be best to just take the remains and freeze them, then bury them appropriately.

In the future to help avoid accidently injuring snakes or other creatures in the grass, maybe poke at the bushes with a stick to scare anything out of them.