take the law into your own hands

to do something illegal and often violent in order to punish someone because you know the law will not punish that person
不透過法律自行懲罰，私下解決

• One day, after years of violent abuse from her husband, she took the law into her own hands.
  在經受了多年虐待之後，她終於有一天私自報復了她的丈夫。