Take the law into your own hands
Definition 1 of 1
Pro Tip 1/3Negative connotation
The phrase suggests disapproval and implies taking action outside the law, which is often seen negatively. 
Vigilantes who take the law into their own hands can end up facing legal consequences.
Pro Tip 2/3Emotionally charged
The phrase often implies acting out of strong emotions like frustration or anger rather than reason. 
Overwhelmed by anger, he took the law into his own hands after his brother was assaulted.
Pro Tip 3/3Not literal
It's a figurative expression and doesn't mean physically grabbing or controlling the law. 
He didn't literally take the law into his hands; he just confronted the thief.