take the law into your own hands idiom
C2 不通过法律自行惩罚，私自治罪
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. 在经受了多年虐待之后，她终于有一天私自报复了她的丈夫。