I seem to remember a recent mall shooter who was stopped by an armed off duty police officer. Does that ring a bell?
Yeah, it was at the Trolley Square Mall, where firearms are prohibited.
There, an off duty police officer who happened to be shopping, used his gun to stop the killer...in that case a lone jihadist.
As far as I am concerned, that police officer was just a US citizen out shopping who happened to have a conceal carry permit as a part of his job. Although that foolish mall policy restricted guns (and it is interesting to note that Utak State law does not...in Utah you can carry in the open by law), a citizen with a gun saved countless lives that day too.
...and that is the point. Armed citizens rise up and stop this kind of thing, preventing the killer fomr just having their way for much longer periods of time where they kill and maim many, many more people.