Liquor store roberies happen every day in this country, frequently they involve the murder of employees or customers. In cities that aknowledge the sanctity of the 2nd Ammendment they don't happen as often, and the bloodshed is more frequently the blood of the robber. But even in those places the worst does happen. Why? Lots of reasons, maybe nobody is armed but the bad guy, frequently people go tharn during a robery, some of us aren't as corageous as we think and when in a mass of people will wait for someone else to be the hero.
It's reasonable to expect that if we allowed armed citizens on planes we'd see the same kind of variations in reaction. Sometimes good, sometimes bad. As a last line of defense that's pretty cool. But as the only line of defense... it's seriously lacking.
And yes, sometimes the dragon wins.