Vigilante

A vigilante (/ˌvɪdʒɪˈlænti/, /ˌvɪdʒɪˈlænteɪ/; Spanish: [bixiˈlante]; Portuguese: [viʒiˈlɐ̃t(ɨ)], [viʒiˈlɐ̃tʃi]) is a civilian or organization acting in a law enforcement capacity (or in the pursuit of self-perceived justice) without legal authority.

https://en.wikipedia.org/wiki/Vigilante