Ideally, a good guy with a gun could stop a bad guy with a gun. But unlike the mythical West, today's bad guy is a psycho with the singular goal of killing as many people before he kills himself. The madman has thus tossed self-preservation out the window, while the 9 to 5 security guard has not. By the time the guard runs to the other side of the building to see what's going on, the madman is already aiming at his next victim. And since he is not worrying about dying, the madman aims better, and might even kill the nervous security guard who fires at random.
Arming more people then is not the answer. It takes a crazy man to kill another human being, and most of us are not that crazy.
But we should be smart enough to keep guns away from those that are -- even if it means keeping them away from the rest of us.