When I was a child I saw a man STOMP a large water mocasin to death.
My Scoutmaster did that once, as well. Water Moccasins are one of those exceptions to the "not generally aggressive" rule, from what I have seen. That, or they are too curious for their own good.
I've killed garter snakes completely by accident with a shovel before. Never even saw them until after they had been severed by the shovel blade. I don't think anyone's too worried about them, though!
FWIW, I was envisioning keeping the muzzle of the shotgun up, rather than using it more like a lever. Still, it would kind of suck to pick that moment to find out your gun was subject to slamfire. It's definitely not a good idea upon reflection...