Thoughts on a cold-blooded snake

I guess I've heard the expression “mean as a snake” all my life, although I honestly can’t remember anyone in particular using it. Maybe it was in the works of southern writers, like Faulkner. He liked saying things like, “Them that’s going, get in the g**damn wagon, them that ain’t, get out of the g**damn way.”  (spoken by Boon Hogganbeck in "The Bear”).

Cottonmouth, Agkistrodon piscivorus is a venomous snake,
a species of pit viper, found in the southeastern United States. 

I imagine some would have described Boon as mean as a snake. I’m trying to “hear” in my mind’s eye the expression being used by my daddy or mother or grandmothers. They were all brought up as Texan southerners in the early part of the 20th century. Both of my grandfathers had died before I was born in 1943. In searching the Internet for an explanation of the origin of this expression, I haven’t found anything that satisfies my need to know. Other adjectives applied to snakes include cold-blooded (which they are), low-lying (which they are), slithering (which they do). Cold to the touch. I don’t care to handle snakes, although they don’t “make my skin crawl”. Now rats, that’s another story. You dirty rat! Snakes do eat rats. I call that justice.