vigilante

Concise Oxford English Dictionary © 2008 Oxford University Press:
vigilante /ˌvɪdʒɪˈlanti/

▶noun a member of a self-appointed group of people who undertake law enforcement in their community without legal authority, typically because the legal agencies are thought to be inadequate.

– derivatives
vigilantism noun.

– origin C19: from Sp., lit. ‘vigilant’.