The tragic story of a shooting spree at a Texas party that left two teenagers dead is precisely why I don't understand the "more guns will keep us safer" argument. The short version of what happened at the party is that some idiot decided to fire off his gun into the air "in a celebratory fashion." Someone else took that as a threat, and fired into the crowd (presumably in the direction of the original shooter). In the ensuing chaos, two innocent people were killed.
That was bad enough. But imagine there were six armed people there, or twelve. And they don't necessarily know each other, and they see other people pulling out guns and firing, and they have no way of knowing who are the bad guys. And so the whole party massacres itself.
Real life, particularly in a civilian mass shooting situation, isn't like Call of Duty. There are no uniforms and nothing glows red when you target an enemy. What more guns means in this situation is more chaos and a situation where not even the cops can tell who is a threat and who is trying to "help".