Heinous tragedies involving mass shootings including the tragedy at the Sandy Hook elementary school

There have been a number of heinous tragedies involving mass shootings including the tragedy at the Sandy Hook elementary school and more recently at the Emanuel African Methodist Episcopal Church in Charleston, the Orlando nightclub, the First Baptist Church in Sutherland Springs, the Harvest Music Festival in Las Vegas, and Stoneman Douglas High School in Parkland. In general, what principles and findings of social psychology could be applied to reduce societal violence? Discuss.