Arizona is a state that allows people to carry guns. Joe Zamudio carries one and was in a store when he heard the shooting and he came out with his gun to see if he could help. What happened then shows that when a gun carrier comes across a confused and fast-moving situations like the shootings last weekend, if they are not very careful they could make matters a lot worse, either by misjudging who is responsible for what and shooting the wrong person or being mistaken as the gunman by police.
This episode suggests that people who carry guns should get at least some of the kind of training that police officers get where you learn caution and never to fire at someone unless you are sure and get some experience in being put into chaotic situations.
Whenever I hear that argument for giving everybody a gun, I think about incidents like this one: http://www.nytimes.com/2009/05/29/nyregion/29cop.html
I was robbed at gunpoint once and shot at twice. In none of those cases would things have turned out better if I had a gun with me.
I’m not sure about Arizona law, but in Ohio 12 hours of training including range time is required to get your concealed carry permit. As a permit holder I can say first hand that permit holders do receive some reining of this sort.