My wife gets on my case when I carry my pistol somewhere. She doesn't like guns. For example, if I carry in a restaurant, her theory is if it is so unsafe that I need to carry my pistol in there then we shouldn't be there. But my point is, it is unsafe EVERYWHERE. Shit like this can happen anywhere. If somebody was carrying in there, who knows. Maybe they could have got a lucky shot off and hit the shooter. Maybe not. But at least there would have been some sort of chance. Even if it saved only one person from being injured, or killed, then the purpose of carrying would have been served.
Of course, gun control nuts will have a field day with this. Guns are no different than knives, baseball bats, alcohol, gasoline, matches, etc. In the hands of the right people who are responsible, there isn't a problem. In the hands of those who want to do harm, they are dangerous. Guns don't cause violence anymore than forks cause people to be fat. It all falls on the person using it.