From Longman Dictionary of Contemporary Englishtake the law into your own handstake the law into your own handsPUNISHto do something illegal in order to put right something that you think is not fair, for example by violently punishing someone instead of telling the police vigilantes who take the law into their own hands → law
Examples from the Corpus
take the law into your own hands• Citizens should not be expected to take the law into their own hands. 