Hey if I wasn't 100% sure I would have killed it also seeing as it was in the yard and then tried to make a better determination. I've killed several Copperheads in my life and make no apologies for doing so. ALL were near the house too close for safety.
Now I feel the same way about other animals as well like coons. I learned a lesson about them. I had a female raising about 5 close to my house. I watched them grow. I also noticed my neighbors were feeding them. The coons soon lost fear of humans and at that point became a danger. I tried to scare them off, baited them with food to an metal pan I had wired with an electric fence surger, and about everything possible. One morning they started growling at me and I said that's it they have to go. The next morning I emptied out the shotgun on them. Not something I enjoyed doing anymore than killing the Copperheads but safety of self and family comes first.
I can identify Copperheads in my region. The problem with Cottonmouths is like Copperheads. They vary in appearance region to region and at different stages of life. Looking at his pictures and not seeing a tail I would have killed it then and there. If I wasn't 100% sure I would have killed it for my families safety sake. I know enough about snakes to realize though that as least as far as Copperheads go a black snake will take one on and likely win.
I leave black snakes alone but in my area all black snakes are safe as we have no Cottonmouths but some swear we have Cottonmouths yet no offical confirmations by a wildlife biologist. I would feel far less remorse over killing a snake I later find out is harmless after more careful identification when it's dead than letting one go that was poisonous and still having it around my home and kids.
Humblegunner did the right thing. If it/was a Cottonmouth there's more near by just like Copperheads. Too if it is/was a Cottonmouth and considerable ways from water something is prompting them to go there like food sources or construction etc. If he were out and about somewhere in the wild or more remote location and saw the snake? Then I would say live and let live and leave it be.
“Geographic Range
Cottonmouths, Agkistrodon piscivorus , are found in the United States ranging from as far north as the James River in Virginia to the western edge of Missouri, and as far south as the Florida Keys and the western part of Texas. They are found in parts of North Carolina, South Carolina, Georgia, Kentucky, Illinois, Indiana, Tennessee and all of Louisiana, Alabama, Mississippi and Arkansas.”
~~~
Watch where ya’ step...;0)
People don't realize that when you feed wild animals, you're endangering them. Was the aggressive raccoon in our backyard semi-tame, or rabid? With all the kids and pets in the neighborhood, there was only one choice to make. He's pining for the fjords now - couldn't take the risk.