Yeah, the cut throats and the pigs. But who wants all that blood spilled, judge, huh? Isn't there a simpler way of not pissing off the big vipers?” – Al Swearengen
From the show Deadwood
What does vipers mean here?
You have to understand it in its context. Earlier, Al Swearengen said:
...First come when I was watchin' you and them lawyers on line this morning. They began to slither in my sight like vipers. So as not to puke, I had to close my eyes. The vision went on. Got worse. I saw the vipers in the big nest in Washington. They were takin' us in the camp for actin' like we could set out own laws up or organizations and then saw the big viper decide to strangle and swallow us up every fuckin' thing we gain here
So, "vipers" here is referring to the judge and the lawyers.
Viper can be "a vicious or treacherous person". It is used here as such, rather than as the animal itself.
The "big vipers" in this context means the "big shots." In a rather pejorative way.