It's high time that interesting word was taken back.
The connotation of "vigilante" has become altogether negative, but the word's history is more complex. Its etymology dates back to "vigilance committees" formed by citizens who banded together to fill a law-enforcement vacuum:
vigilante "member of a vigilance committee," 1856, Amer.Eng., from Sp. vigilante, lit. "watchman," from L. vigilantem (see vigilance). Vigilant man in same sense is attested from 1824 in a Missouri context. Vigilance committees kept informal rough order on the frontier or in other places where official authority was imperfect.
The 1856 origin above clearly refers to the San Francisco Committee of Vigilance, organized in that year to deal with general corruption in the city and the chaos following the rapid population growth resulting from the influx of 49er gold seekers. Violent crime and corruption were rocking the city.
As for local conditions, it is enough to note that in the first ten months of 1855 there were 489 murders in the state and only six legal executions. Stuffed ballot boxes were used to qualify the election of supervisors who did not reside in the districts voted. Ballot boxes with false bottoms were common.
There were actually two Committees of Vigilance. The first, formed in 1851, was an impromptu assemblage of locals, while the second Committee was more organized and published a statement of intentions and had more than 700 members who openly identified themselves.
There are dangers with do-it-yourself justice, of course. The Ox-bow Incident (1943) was an excellent film showing citizen law-enforcers run amok. In the dark western, an innocent Dana Andrews was accused of cattle rustling and hanged by an overly zealous posse, despite Henry Fonda's misgivings.
Even the real-life San Franciso Committee of Vigilance was known to stretch a neck or two in its crime-fighting activities.
Of course, the Minutemen are not lynching Mexican nationals or anyone else. They are armed with cell phones and binoculars and are more accurately understood as a neighborhood watch with 30 SPF sunblocker.