To do something illegal and often violent in order to punish someone because you know the law will not punish that person:
One day, after years of violent abuse from her husband, she took the law into her own hands.
Cambridge Dictionary
Impersonating A Cop Hidden Camera Prank
5 mrt 2011
A criminal dressed as a cop asks for help getting out of his parol car and then runs away as his unwitting accomplices feel terrible about what they just did.