Did you check the mouth for fangs?
That head doesn't look like a viper's head, too rounded, but really easy to confuse them, as a lot of non poisonous snakes have the appearance and general shape of poisonous snakes found the same regions. King snakes/coral snakes are a good example. Rat snakes and bull snakes have a similar appearance to rattle snakes, etc.
Most adult cottonmouths are very dark brown or even black colored, with the bands gone or almost gone on most of their bodies. Nasty tempered bastiages for sure. Cottonmouths will charge and chase people/animals, where as most other snakes flee when given the opportunity. I guess Cottonmouths believe in the castle doctrine, vs duty to retreat
I probably would've shot the bastiage too, or at least smacked it good with a big stick or shovel.
I killed a copper head here in Oregon once while out fishing. It was waaaaaaaaaaaaaaay far away from it's normal habitat, and the only thing I can think of as an explanation is that it came in on a truck from somewhere in the mid west or down south OR someone was keeping one as an exotic pet, didn't want it anymore, and dumped it in the woods. Pretty damn scary to find one of those around here though. I've never encountered a rattler - been buzzed by them but never saw them - but finding the copperhead was pretty damn spooky. Took a couple blows from 10-20lb rocks to kill the damn thing.