I was not under the impression that this was a common view. For example, in films and media that depict school shootings, the shooters are usually depicted as regular kids that get pushed to the limit by bullying and abuse. Besides gun control, mental health is often the focus when speaking about school shooters, but it is more about basic and more general aspects of mental health rather than the “psychosis and other serious psychiatric illness” mentioned by the article. I also see this in the news when discussing this type of event - the emphasis is rarely on the shooter’s severe mental illness, but rather on how they were (mis)treated, abuse that they might have faced, other life history, and less severe metal traits such as narcisim and suggestibility.

Also, according to the article about 20% of school mass-murders with firearms and 32% of those without firearms were perpetrated by people with severe mental illness, so severe mental illness is responsible for a good amount of these.