OK, here's my take on it. I think the school shooter basically has a personality disorder based on avoidant insecure attachment.
First some background psychology:
Attachment is the psychological bond that an infant forms with the parent/caregiver. It is a strong, wired-in instinct, that is essential for survival (in animals we call it 'imprinting'). The caregiver provides a 'secure base' for the infant, from which it can explore and cope with the world; it likes to stay physically close to the caregiver and retreats to it for support and comfort when it feels anxious and insecure. As the infant grows up into independent adulthood, the physical (and psychological) distance between it and the caregiver increases until it can largely function on its own (for that it has to negotiate various challenges in development, see
Erikson's stages of psychosocial development), although its attachment experiences will determine the type of relationships it is able to form with other people, and indeed a partner. Attachment behaviours emerge particularly strongly when someone is trying to cope with feeling under threat, anxious, stressed, unwell or insecure.
There are roughly three flavours of attachment pattern:
Secure Attachment: "I know that I matter and can therefore trust my caregivers to be consistently there for me;
Insecure Anxious Attachment: "I'm not sure that I matter because I cannot trust my caregiver to be consistently there for me; therefore I better work hard at
making them care. This becomes the sickly, clingy, dependent child, or the child that becomes very good at meeting the needs of the caregivers (the 'parentified' child, or the co-dependent carer), because the best way to make sure you matter, and to get their attention is to meet
their needs.
Insecure Avoidant Attachment: "I'm not sure that I matter because I cannot trust my caregiver to be consistently there for me; therefore I better learn to stand on my own two feet as quickly as possible. This becomes the introverted, apparently very self-contained but really rather fragile child, distrustful of and hostile towards others it feels rejected and abandoned by. Because of its fragile insecurity, it really wants nothing more than to get people's love and attention, but its fear of rejection/abandonment is so great that it can't let anyone get close. Thus a push-pull (approach-avoidance) relationship dance begins.
Insecure Disorganised Attachment: a chaotic mixture of the two. These children become Borderline Personality disorders: others are perceived either all good (perfect parents) and relationships are based on cloying adoration and dependence, or all bad (abusive monsters), with relationships based on hostile rejection and abuse/neglect.
Now, moving to the school shooter.
School shooters, I predict, are avoidantly attached. They crave attention and acceptance, but they expect rejection and betrayal. Therefore they remain remote and distant whilst simultaneously trying to garner attention, and grimly collect "proof" (in Transactional Analysis terms, "trading stamps") for their beliefs that people will reject them (this is the "cynical" bit alluded to by the authors). Often they adopt controversial behaviour or appearance for both these purposes of attention seeking and inviting rejection. On those occasions that they do get love or attention they find it difficult to cope with (never had an example from their parents, see?) and test the relationship to breaking point --and when it inevitably does, they feel proved right in their worst suspicions of betrayal (often
way beyond what actually happened) and go off into a hostile rage. In the field we call this: "working towards a tragic outcome".
All this is fueled by a shatteringly low self-esteem. Like many avoidantly attached people, school shooters may adopt a Narcissistic defense against this low self esteem, arguing with themselves (and anyone who wants to hear it) that it's not them who is lacking, but the callous, rejecting world full of self-obsessed superficial morons. They project the qualities they perceive in themselves on the world; despise weakness and emotional need, glorify strength and emotional detachment. Fascist ideals often appeal to them because Fascism is after all basically one big group/tribal Narcissistic defense... Of course, at some point the school shooter has collected enough "trading stamps" and feels the need to prove his superiority once and for all. Usually this is after an escalation of significant events --rejection by one too many girl, for instance, or repeated ridicule by someone of perceived higher social standing amongst peers, and after weeks to months of fantasy revenge rehearsals. Then one day Johnnie gets his gun... and the rest is history.
School shooters are generally loners, but like any birds of a feather, may flock together. Although they struggle to have relationships, they can manage relationships that feel "safe" because the other person feels as low in self-esteem, as much of a misfit as they do. In their little club, they feel validated and accepted in their distorted world view and can egg each other on to the eventual tragic climax.
Throw into that the Eriksonian development issues that are important from early childhood up to teenage/college years, (developing purpose and morality, competence and achievement, social skills and intimacy) and you get a more detailed picture of how a school shooter is formed.
See? It's no big mystery. "Cynical Shyness" is just a new, oversimplified name for existing concepts and models that have existed for up to almost a century and every competent psychologist knows about.