Jared Louchner at the Tucson shooting was stopped by an Arizona CCW holder.
Major Hassan, the jihadist at Ft. Hood, was shot and wounded by an armed woman MP. Those who were shot and killed were NOT ARMED.
Lesson learned. Carry your weapon when on duty, and if allowed to, when off duty. You become the first line of defense for the unarmed, intended victims, and often are the ONLY LINE OF DEFENSE for them.