I know cottonmouths!
Not to be a "correctibutimus", but Imur, you had a mean snake, period. No egg guarding, rattlers(actually all pit vipers) give birth to live young and wouldn't give a rats' fuzzy butt if you methodically skinned their babies alive in front of them!
As for the cottonmouth, I've run across more than I can remember; being a snake catcher and fisherman growing up in the south. Call me lucky, but I once caught one at a young age thinking it was a watersnake. The watersnakes around these part (Neriodia genus, formerly Natrix) are nervous as a bunker full of Sadam look-alikes and I'd been chewed up before, so tried to pin them down and grab their heads. This snake never opened it's mouth as I've seen them do, just held still on the bank. I pinned it down, grabbed it's neck and carried it home and put in a cage. My Dad came home from work, I proudly showed it to him, and he had a fit dumped it out and killed it in the yard with an axe handle.

He later opened it's mouth and lifted it's fangs with a twig, smacked me on the back of my head and told me what an idiot I was!
I learned the difference, although a big northern water snake is just as stout and it takes a good look! But having caught cottonmouths (on purpose) since, I must say the tempermant isn't much different than a copperheads', if you factor in that you have to get a stick under them and sling them away from the water to have a prayer to catch them. I've never upset a copperhead that much (well, off the bat), but can imagine they would react similarly to it! They are the same genus, after all. But look at corns docility compared to most other ratsnakes, and that nullifies my arguement!
I seriously beleive that native U.S. snakes that eat fish are MEAN, period. Garters don't count, they're omnivores! But every water snake I ever caught was unhandleable and cottonmouths will strike a foam covered PVC piece ruthlessly.
I saw the bit on poisonous snakes being tested for aggression, too and was amazed how calm most were. I'll do the "foot press" test next time I run across one on a snake hunt and let you know.