I think this is relevant to the thread.
The Seven Myths of Mass Murder By J. Reid Meloy, Ph.D.
J. Reid Meloy, Ph.D. is Clinical Professor of Psychiatry at the University of California, San Diego, and President of Forensis, Inc., a nonprofit dedicated to forensic psychiatric and psychological research.
Myth 2: They can easily be divided into “psychopaths, psychotics, and depressives”
David Cullen, the journalist and author of Columbine, an excellent book on the high school mass murder in Colorado in 1999, has asserted this formulation. Unfortunately, his diagnostic classification of mass murderers is much too simplistic. Most are complex in their motivations and psychopathology. They often have both mental and personality disorders.
Mental disorders range from chronic psychotic disturbances, such as paranoid schizophrenia diagnosed in the Jared Loughner case, to major depression, bipolar, and other disorders. This may sound like splitting hairs, but when it comes to risk mitigation, fully understanding the range and complexity of these individuals’ disturbances is critical.
Personality disorders also abound in this group and are often a mixture of antisocial, paranoid, narcissistic, and schizoid traits — or in layperson’s terms, someone who habitually engages in criminal behavior, is suspicious of others’ actions, is self-centered and grandiose with little empathy, and is chronically indifferent toward others and detached from his emotional life. What Cullen has done is a disservice to the millions of individuals who are clinically depressed or have a psychotic disorder and pose no more risk of violence to others than your neighbor. Loughner has given paranoid schizophrenia a bad name — many other factors contributed to his attempted assassination and mass murder.
Myth 5: Psychotic individuals cannot plan in a precise and methodical manner
The majority of adult mass murderers are psychotic, meaning they have broken with consensual reality and perceive the world in an idiosyncratic and often paranoid way. Yet they may research the internet for weapons, practice video games to sharpen their marksmanship, purchase weapons and ammunition, conduct surveillance of the target, and carry out their mass murder, all from within a delusion.
A delusion is a fixed and false belief and may provide a rock-solid motivation for mass murderers. Paradoxically, delusions may help them commit irrevocably to paths of homicidal destruction. Our research has also found that mass murderers who are psychotic have higher casualty rates than those who are not. Typically they select victims who are complete strangers, who in their minds make up a “pseudocommunity” of persecutors bent on their destruction.
The seven myths of mass murder | OUPblog