Modern society has repressed the traditional role of men in society as protector. This leaves the to men the more natural and primitive role of predator.
Also the modern world has changed the traditional Anglo-Saxon Protestant ideal of male self effacing humility to one of self aggrandizement.
Also gone is the concept of honor. In the distant past men would fight to the death to defend the honor of their Lady or themselves. Today the mere mention of the concept is laughed at derisively.
Gone as well is the Christian ethics that used to help keep men's aggressive tendencies in check.
Modern society's total capitulation to Feminist narcissism has destroyed all of the traditional checks on male's aggressive nature while promoting feminine traits as the ideal. So what we are left with is either males that have become semi-masculine women or males with no inhibitions to their natural state masculine nature.
Frustrated predator. The most overstuffed cat will still engage in hunting behavior. I'm guessing none of these mass shooters were ever deer hunters.