Since you posted this in two different threads, I'll respond to it here as well.
According to several reports, the shooter in that case saw the subject of your story pointing his weapon at him and decided to end his own life at that point.
In the other cases I cited earlier, armed citizens confronted the shooter with their weapons and were able to stop and hold them without resorting to firing. There is no doubt that the mere presence of weapons has an effect on criminals.