Ultimately, of course, this is all just a bunch of people being brave in hindsight. We know that guns are effective at deterring normal crimes, however an insane shooter obviously offers a different problem. At some point a shooter is going to run up against an armed citizen, and then we'll find out for sure just how effective they will be. Hopefully that armed citizen is responsible and capable of using their weapon effectively and are not just carrying around a gun to feel safe.
There are many incidents where an armed victim defeats unarmed or armed criminals. Here is one school shooting that was stopped by armed students:
Unfortunately, the only incidents of pistols vs body armor + rifles ended poorly for the pistol users.
FWIW, I do believe guns can be, and are, legitimately useful in self defense; however, in the scenario of pistol vs body armor + rifle, the pistol user is at a very, very serious disadvantage that can only be overcome by a very high level of still and a good dash of luck.