My colleague James Alan Fox and I have studied hundreds of cases of mass murder, including those committed at random in shopping malls, churches, health clubs, and cinemas.

Almost every killer who opens fire indiscriminately in a public place seems to have been delusional, paranoid, and hallucinatory.

Yet no matter how definitive the evidence, this conclusion does not sit well with Americans who seek justice for seemingly “crazy” acts of violence. It seems to me that many people would prefer to believe that mass killers are evil rather than sick.