This one was uttered by an evangelical Trump supporter. It is a congruent conflation of “a nest of vipers” and “a den of thieves”, both meaning a group of individuals suspected of underhanded dealings. “Den of iniquity” (a lot of immoral things happen there) might be in the mix, but I doubt it. “Waliking into the lions’ den” (place yourself in a dangerous situation) certainly is in play given the context and its Biblical roots. Here is the article where the malaphor is found: https://www.washingtonpost.com/opinions/2019/08/14/evangelicals-view-trump-their-protector-will-they-stand-by-him/?wpisrc=nl_rainbow&wpmm=1
A big thanks to Barry Eigen for spotting this one!