Keep this one thought in mind, the best argument against armed citizens:
NPR's Robert Siegel talks with Stanford University law professor John Donohue about his research on whether or not increased gun ownership helps decrease crime ... we were looking at data across all 50 states and the District of Columbia over the period from 1977 through 2014.
The basic finding was that the net effect of allowing citizens to carry concealed handguns was an increase in violent crime, which essentially rose to about a 15 percent increase after 10 years of existence of the right-to-carry law ... the single biggest effect seemed to be an increase in aggravated assaults. And also because increased right to carry does complicate the task for police and therefore sort of serves as a impediment to good policing. And the one thing we know is good policing is probably the single most important thing in curtailing crime.
The interesting thing is the FBI did a very intensive study of 160 mass shootings over the period from 2000 to 2013. And what they found was that over that period, in the 160 cases, there was only one incidence of a private citizen who was not security personnel or a police officer who effectively intervened in the mass shooting, and that individual was an active duty Marine. On the other hand, 22 unarmed citizens intervened to stop those mass shootings, typically when the individual was reloading. And so it gives you a sense of the relative effectiveness of relying on someone with a gun to intervene in an active shooting scenario.