vigilante

noun

1. A member of a vigilance committee.
2. A person who acts outside of legal authority, often violently, to punish or avenge a crime, right a perceived wrong, etc.

   “For example, a voice accusing a man of being a paedophile might be based on recent TV news programmes about antipaedophile vigilante groups, plus the memory of having been accused of being a pervert by a neighbour, plus fears that he might be a secret paedophile because he enjoyed reading Lolita.”

   “Jeptha Billingsley, who was no prize pig himself (he had been arrested for horse theft and assault with intent to murder), was to be taught a lesson, and the vigilantes made him cut down the McLemores.”