Question

A blank is a person, especially the member of a group, who takes law into his own hands

Answer 1

I believe it is vigilante

Although I am not quite sure because I do not know the background context to this question.

Answer 2

Answer:

Vigilante.

Explanation:

Vigilante is a form of the word vigilant, which means "keeping a watchful or close eye on events and people." Sometimes a vigilante will make news for catching a criminal, and sometimes vigilante groups form to target crimes in a bad neighborhood. A common combined use is "vigilante justice," which refers to a form of law-keeping outside of the official channels of police and the courts.

Hope this helped! :)