let the devil out

verb2

1. •
   To behave badly or cause to behave badly.

   • He paused a moment to let logic sink in. Then he let the devil out. “You don't, I'm gonna punch you in the face, piss in your lobby, and shit in your kitchen..." — 2009
2. •
   To ward off bad luck.

   • After fifteen minutes you had to pierce a hole in the back of each shell, to let the devil out and the sauce in', and then you had to rinse them clean in the water in which they had boiled. — 1994