Vigilantes are persons or groups who take it upon themselves to enforce the law (or their interpretation of it) without proper legal authority, and to summarily punish wrongdoers. In western society, it is almost universally discredited by official law enforcement, because of the lack of due process, citizen rights, or formal processing necessary in any society with an organized legal system.
"Vigilante" is a Spanish word meaning "guard". The term was popularised in the American frontier towns of the mid-nineteenth century, especially during the gold rush periods, when settlements sprang up more rapidly than legitimate law enforcement could be provided to them, and citizens instead elected "vigilance committees" to settle disputes and punish lawbreakers. Contrary to popular belief, many of these early vigilance committees were fairly effective in providing a rugged version of due process, law and order where the federal government was temporarily unable to.
While the earliest frontier vigilance committees were elected from respectable citizens, later ones tended to be secret societies who took the initiative by force, their members often wearing masks or hoods for anonymity. In the Reconstruction era southern states, a new kind of vigilantism emerged, with Confederate loyalist guerilla groups such as the Ku Klux Klan enforcing their racially and politically-motivated interpretation of justice by intimidating or lynching liberated slaves or Yankee interlopers.
 Modern vigilantism
Modern vigilante movements are generally the result of people feeling as if the official system is letting them down. Gangs, for example, generally enact a form of justice from within their own groups. Lynch mobs, successful or otherwise, often feel as if a criminal "got off to easily", or feel someone who the law does not or cannot arrest is clearly guilty of the crime and should be arrested.
The minuteman movement acting as "boarder patrol agents" was stared in an atmosphere of paranoia or moral panic spurred by politicians around the illegal immigration issue in states that boarder Mexico. The assumption was that the government was not doing enough, so the citizens would "take it into their own hands". Many problems - often involving violence -have arrisen from this desire to do it themselves.