It's interesting that you quote that shooters "claimed" and "experienced feelings" of being ostracized, because many of them do indeed feel that way, like Elliot Rodgers thinking of himself as a perfect gentleman passed over by women for some incomprehensible reason despite clearly not being one. The catch is that those feelings are not always rooted in reality: many of them were well-liked and popular. What they also often turn out to be, though, is "injustice collectors" who tend to hold grudges and refuse to take responsibility for their own shortcomings (including, I assume, their own contribution to being excluded by others), according to this FBI report: https://www.fbi.gov/file-repository/stats-services-publicati...