1) experienced violence before, often early
2) sometimes experienced psychological problems, maybe as a result of 1)
3) sometimes had some reason to become angry (before the mass-shooting, such as getting fired)
4) sometimes explore/study other mass-shootings before committing one themselves
5) had the means to carry out this crime
Ok, with a bit of flexibility (other than point 5) that could describe every last one of my coworkers, for none of which there is any chance whatsoever they'll ever commit a mass-shooting. The idea is ludicrous.
Typical psychological social-science "insight".