All good points that need to be echoed, save one.Imagine a concert hall with hundreds of armed spectators. The attackers armed with semi-automatic weapons enter from the rear of the auditorium and begin firing. In this chaotic scenario, trying to identify the perpetrators and returning fire requires special police or military counter-terrorist training, which none or very few of the "good guys with guns" have. In the utter confusion and panic, these armed civilians simultaneously begin returning fire. How many innocent attendees will be hit by bullets from untrained "good guys" intended for the attackers? Since you are using your weapon, how can you tell if some other civilian might mistakenly identify you as a terrorist and start shooting at you? The possibilities for casualties from friendly fire increase exponentially in direct proportion to the number of untrained shooters misidentifying the perceived threat, or from those who simply do not practice aiming and shooting regularly on a target range. You may be a crack shot and are trained to keep a cool head and zero in on the actual targets, but how many among hundreds of members of the audience can do the same amidst the mayhem? You might as well set off dozens of hand grenades in the concert hall, since that would be the net effect of hundreds of panicked, untrained civilians "defending" themselves from the attack.I do not think you have thought this through very carefully.
I am interested in the idea of armed civilians posing a threat during a terrorist attack. Has this actually happened, or is this just a theory? Let's consider another theory: Some bad guys are shooting innocent people and I stand up with my licensed concealed carry gun and drop one or two bad guys before someone misidentifies me as a bad guy - because I'm also shooting. Then someone drops me. Know what? I'm OK with that. I've given my life to stop brutality. People of courage have thought carefully about their values, thank-you.