It's not wild generalisation. Mass shootings are frequent. There have been 22 school shootings in the US so far this year alone, and many, many more thwarted attempts. Only one shooting has made the international news.
The FBI has had the opportunity to do extensive profiling of shooters.
In the case of students in a school, the following factors constitute a profile -
Intrusion of preoccupation with violence into schoolwork, into conversation, offhand comments, assignments, jokes, SM.
Low tolerance for frustration.
Poor coping skills.
Lack of resilience.
Inability to cope with rejection or perceived rejection.
Ruminates over 'injustices' he has suffered.
Undiagnosed depression giving rise to extreme emotions.
Narcissism.
Others are dehumanised.
Alienation, a feeling of being different.
Lack of empathy.
Feeling of entitlement.
Attitude of superiority.
Pathological need for attention.
Externalises blame.
Masks low self esteem.
Anger management problems.
Intolerance/prejudice, racism, misogyny, prudishness, bigotry.
Inappropriate humour/belittling, insulting, mean.
Manipulative.
Low level of trust in others/suspicious of others.
Closed social group/echo chamber.
Behaviour is confrontational, reckless.
Rigid and opinionated/mansplains, disregards facts, reasoning against his opinions.
Unusual interest/preoccupation with sensational violence and incidents of this.
Fascination with violence in media/online.
Negative role models.
Student falls down rabbit hole of preparations for violent act.
Families tend to circle the wagons around the student when school communicates about problems despite often being dysfunctional or not showing signs of really valuing family members.
In the case of adult shooters (loosely defined as people over 18), the FBI has developed the following demographic profile -
White male.
Aged 18-29 or 40-49.
High school education or dropped out of university/college, or attended trade school.
Single (no current or previous long lasting relationship) or divorced/separated.
Not necessarily complete loners or socially isolated. Many lived with family/extended family.
Context -
Preparation - most prepare well in advance, leave evidence of intentions which is later discovered (SM, writings, trail of websites visited).
Gun purchased legally or already legally owned.
Stressors that the shooter does not cope with. (See all the school student problems in responding to various stressors).
Mental health problems. Mood disorders, anxiety disorders, personality disorders, psychotic disorders have been diagnosed in shooters before they committed their crimes. Most do not have a diagnosis until assessed afterwards.
Others noted worrying speech or written communication on the part of the shooter at least but mostly more than a week in advance of the shooting.
Personal observations after the fact, from friends, family, co-workers, with the majority of observations dating more than six months before the attack, four or five behaviours observed in each shooter -
Possible mental health issues.
Intrusive remarks and clear preoccupation with violence, grievance, stressed - mentioned to others.
Quality of thinking/communication. Not logical, refusal to see sense.
Anger. Quick temper.
Abuse, harassment, bullying.
Physical aggression.
Risk taking.
Firearm behaviour. Preoccupation, recklessness, glorification of guns.
Violent media use.
Weight/eating. Changes to norm.
Drug abuse.
Impulsivity.
Alcohol abuse.
Physical health.
Fascination with violent role models, or criminals, or organisations, e.g. SS, or martial spirit.
Sexual behaviour.
Domestic abuse, stalking.
Sleep quality/hygiene/appearance.
Personal grievances and personality problems are topped up with strong feelings on political topics in cases where a shooter has a clear racial or sex-based motive.
Yes, access to guns is the crux of the problem.
Such is the culture and political climate of the United States that nobody is ever going to enact comprehensive gun control.
Only thirteen states have laws allowing police to remove a gun from the possession of an individual determined to pose a threat to public safety.
Plus, the FBI has observed that moat shooters do not have an extensive recorded criminal history when they head out to buy the weapon they are gong to use in the attack. Running a criminal background check is therefore not going to flag anything.
A problem with the FBI stats is that domestic violence is vastly underreported and rarely prosecuted, leaving no record. Many families normalise a level of verbal, emotional, psychological, and even physical abuse, and minimise the threat a shooter poses to their safety, even though more than half of shooters kill a family member as the initiation of their spree.
The problem with flagging mental health issues is that individuals who end up firing indiscriminately into a school or supermarket or workplace are people who have never seen a MH HCP. The MH issues become apparent in the violence itself and when family and friends put clues together, and when the individual is interviewed by police and assessed.