A vigilante is a civilian or organization that undertakes law enforcement (or actions in the pursuit of self-perceived justice) that is without legal authority.
I like Batman, and the ideas that Robin Hood of Locksly stood for. However an armed militia was never intended to try to keep the current political system, yet circumnavigate the ruling of its courts. Simply put the Constitution of the United States allowed for an armed militia. And that militia should stand ready to defend the citizens when the U.S. goverment gets to big for its britches. But no militia should act in a way that supports 50 or 60% of the goverment, and tries to reform the remainder of the goverment with violent insurrection.
A good rule of thumb to follow is that a militia should only pick up arms to either help defend the goverment, or to compeletly destroy it and that militia should help a new goverment be established.
Vigilanties want to create problems within a goverment structure. (also without some sort of moral values a person can not be involved with a legitimate militia) atheists, and agnostics can only be rebels without a cause and that is a vigilantes.