Thoroughbreds
One strong common factor among the recent school massacrists: All come from upper-class or professional families. This isn't new. Several episodes of old cop shows like Dragnet and Highway Patrol deal with similar massacrists, and they're always from upper-class homes.
The class connection is obvious. Rich families can pay teams of top shrinks, and can afford low-security private asylums. Indulgence is more important than real cure.
Maybe there's something else going on. How do modern rich fuckheads select mates? They're looking for
high IQ and low empathy, plus infinite greed and zero resilience. Those are the qualities needed for success in NY or DC or LA or London. And what happens after several generations of inbreeding, always trending toward higher IQ and lower empathy and more-than-infinite greed and less-than-zero resilience? You asked for it, you got it.