"Mental illness" is broad, vague and can encompass pretty much everyone at some point in their lives.
Someone suffering from depression over a loss is technically experiencing mental illness.
Mental illnesses are health conditions involving changes in emotion, thinking or behavior
(or a combination of these). Mental illnesses are associated with distress and/or problems
functioning in social, work or family activities.
- Nearly one in five (19%) U.S. adults experience some form of mental illness.
- One in 24 (4.1%) has a serious mental illness*.
- One in 12 (8.5%) has a diagnosable substance use disorder.
Mental illness is treatable. The vast majority of individuals with mental illness continue to
function in their daily lives.
https://www.psychiatry.org/patients-families/what-is-mental-illnessI think the article is focusing too much on the word "illness" as if mass murderers who have no history or diagnosis of mental illness makes them normal.
As we all learned, criminals often plead "not guilty by reason of temporary insanity" to avoid a life sentence or the death penalty. By definition, mental illness can be temporary ("is treatable") and can therefore be a one-off reaction to some event. Still rises to "mental illness".
As for school shootings, without prior diagnoses or therapy and/or evidence of mental illness from family and friends, it's kind of hard to diagnose them after they are DEAD.
Rational, healthy-minded people do not commit suicide unless you want to count the exception of assisted suicide. Unless these school killers are terminally ill, it's impossible to include assisted suicides in this discussion.
Too many students have committed suicide for the same reasons the killers give for murder -- bullying, failed romances, being "weird" and having no friends, etc. The difference is the killer took a few students with him on the way.
Too many people in academia like to excuse aberrant psychological behavior because they know it's impossible to identify and treat everyone that commits violence BEFORE they do it. Denying they were mentally ill at all absolves them and their profession of doing what they often say they can -- diagnose and treat people with mental illness before it causes someone to get hurt. So much easier to say the killers are mentally okay making them undiagnosable.