There is a difference between vigilantism and mutual self defense. Putting riflemen on top of schools and shopping centers (or around them) would not be vigilantism. Even a posse, organized by and/or operating under the auspices of the local Sherriff, would not be vigilantism. It would however be anti statist, and so is quite unlikely to occur, at least in those parts. If had been in much of flyover country, there would already be citizen patrols, on horseback, 4WD, and on foot as required, searching for these scrotes. It's what's known as the posse. It goes back to England, and other countries also to some extent, well before any English colonies existed in the New World.