It's been a year since the Gabby Giffords shooting in Arizona. I can't believe how many times I've read posts related to this shooting (and others) where folks have said something like "If there had just been one good person there carrying a gun... things would have been different. Well, while they certainly might have been different, there is no guarantee that it would have been different in a good way.
It's apparent that most of the people commenting haven't actually been on scene when a shooting incident was taking place, especially in crowded public places. There is chaos. People are diving to the ground; running in every direction... Unless you are extremely close to the shooter, the odds of you seeing him/her long enough to get a shot off are small, and the odds of you having a clear field of fire (no one behind the shooter) are even smaller. Even then, there is a good chance that someone will run in front of you, or bump into you.
But let's say that you do get a clear shot with a clear field of fire without being jostled in the process. The shooter goes down. But in the crowd is another person carrying a gun, and all they see is among the chaos someone taking a shot and another person going down; will they mistake you for the shooter and also take a shot... at you?
What if it is a policeman responding to an "active shooter" call arriving on scene just in time to see you take the shot, and another person (the real shooter actually, but the officer doesn't know that), goes down. Will you end up in his/her sights?
How many people have really thought through the situation? Sure, if the shooter is aiming at you (or a love one), or is swinging his/her gun in your direction, the situation would warrant a response. But other than that, there is much to consider and little time to do it unless you've thought it through thoroughly beforehand and understand the risks. Yes, things might have been different; but maybe not in the way you might expect.