This is an article from National Review using excerpts from a Gladwell article from a few years ago. Basic premise is with each successive shooting the "social threshold" lowers for the next individual. He points to Columbine as the major pivot point. Deeply interesting and disturbing at the same time.
https://www.nationalreview.com/corner/why-do-mass-shootings-happen-best-explanation/
https://www.nationalreview.com/corner/why-do-mass-shootings-happen-best-explanation/