I am going by this from Smithsonian. Mass murders in the past were mostly organized crime fighting over turf.
https://www.smithsonianmag.com/history/story-first-mass-murder-us-history-180956927/
Yeah, In the Army they wanted us to get in a fox hole and cover ourselves with a poncho if we were nuclear attacked. I always said, “They want us to do this so that we will be easier to bury!” :-)