A nest of vipers
A group of iniquitous people, congregating together.
'A nest of vipers' is a figurative term but it isn't very far removed from the literal meaning. Calling someone a 'snake' has long been an insult in English. This derives from the ingratitude or treachery displayed by the snake in Aesop's fable The farmer and the snake, in which the snake was rescued from freezing to death by a farmer, only to mortally bite him when it recovered. The secretive habits of snakes also gave rise to a reputation for lurking danger and to suspiciousness. Vipers - also called adders - were especially singled out as despicable because they were, unlike other English snakes, poisonous. To call someone a viper was to insult them considerably. This form of insult dates back at least to the 16th century. William Tyndale used 'vipers' as a figurative insult in his 1526 Bible, in Matthew 3, :
He sayde vnto them: O generacion of vipers, who hath taught you to fle from the vengeaunce to come?
Not long afterwards Robert Greene used the term even more forcibly in The Art of Conny Catching, 1591:
These villanous vipers, vnworthy the name of men, base roagues,... being outcasts from God, vipers of the world, and an excremental reuersion of sin.
Groups of people, usually those of villainous intent, were called 'nests' from around the same period. The first documented occurrence of the two terms combined to form 'a nest of vipers' was in 1644, when a pamphlet that criticised a group of plotters who were planning treason against the English Parliament was titled A Nest of Perfidious Vipers.