Definition - What does Vigilante mean?
A vigilante is a person who sees it as their own personal duty to enforce just measures against people who have acted in an unjust manner. More specifically, a vigilante does not act under any legal authority when taking actions against who they perceive as someone to break the law.
Justipedia explains Vigilante
A vigilante is nearly always motivated by the rationale that the current means of legal authority do not do an adequate job in punishing those who break the law or that the law itself does not punish for something that the vigilante believes should be punishable. This type of mentality has existed among groups of people dating back through history. One of the most violent vigilante acts in United States history is known as "The Great Hanging at Gainsville."