Body armor does a great job of keeping you alive, and a shit job of keeping you comfortable. It's incredibly hard to continue whatever you're doing while you're being punched repetitively in the center of mass. If a "John Wayne wannabe" would have knocked the breath out of, or even just distracted, the shooter, then there would have been an opportunity for him to be attacked. Your assumption that there is nothing that can be done about an armored attacker with a gun is most likely the product of your unfamiliarity with guns and does not reflect reality.
Furthermore, your ignorance of the subject is showing, the Colorado shooter had no automatic weapons. You were probably confused by the term "assault rifle" which is commonly assumed to mean an automatic weapon, but in fact (in the US) is legally defined as a weapon that has a detachable magazine and at least two of several cosmetic features such as a forward grip or a barrel shroud. This is why assault weapon bans are commonly ridiculed as bans on scary looking guns.
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.
