See Slate's complete coverage of the Gabrielle Giffords shooting and arrest of Jared Lee Loughner.
That's what happens when you run with a firearm to a scene of bloody havoc. In the chaos and pressure of the moment, you can shoot the wrong person. Or, by drawing your weapon, you can become the wrong person—a hero mistaken for a second gunman by another would-be hero with a gun. Bang, you're dead. Or worse, bang bang bang bang bang: a firefight among several armed, confused, and innocent people in a crowd. It happens even among trained soldiers. Among civilians, the risk is that much greater.
We're enormously lucky that Zamudio, without formal training, made the right split-second decisions. We can't count on that the next time some nut job starts shooting. I hope Arizona does train lawmakers and their aides in the proper use of firearms. I hope they remember this training if they bring guns to constituent meetings. But mostly, I hope they don't bring them.