I usually try not to weigh in on stuff like this, but i have a personal story to tell about it. You might remember a few years ago that some kid shot up a church group in Denver, then went down to Colorado Springs the next day and shot up a Big Church there. Someone in that congregation WHO WAS ARMED scored a couple hits on him before he turned the gun on himself. In this particular case, it was someone in the congregation WHO WAS ARMED who stopped the carnage.
Shocker. It was even more shocking to me because, while he was on his way down to the Springs, a phone in our house rang. Yes, we were one of the people that he called on his way to his final scene.
Did we have any idea that he was going to do this? Nope. When we spoke to him on the phone, he said he was going away and we probably wouldn't see him again. In 20/20 hindsight, maybe we missed a signal, but people don't go off and do this kind of thing. Sure, there have been a number of incidents like this, but in a nation of 300 million people it's a VERY SMALL minority who go off the rails and do this sort of thing. Most people who have guns are responsible with them.