So, what role does mental illness play in these mass killings?
Multiple studies done between 2000 and 2015 suggest that about a third of mass killers have an untreated severe mental illness. If mental illness is defined more broadly, the percentage is higher. In 2018 the Federal Bureau of Investigation released a report titled “A Study of the Pre-Attack Behavior of Active Shooters in the United States Between 2008 and 2013.” It reported that 40% of the shooters had received a psychiatric diagnosis, and 70% had “mental health stressors” or “mental health concerning behaviors” before the attack.
Most recently, in July 2019, the U.S. Secret Service released its report “Mass Attacks in Public Spaces—2018.” The report covered 27 attacks that resulted in 91 deaths and 107 injuries. The investigators found that 67% of the suspects displayed symptoms of mental illness or emotional disturbance. In 93% of the incidents, the authorities found that the suspects had a history of threats or other troubling communications. The results were similar to those of another study published by the Secret Service on 28 such attacks in 2017.
[...]
It should be emphasized that mentally ill patients who are receiving treatment are no more at risk for violence than the general population. Yet it is also clear that without treatment some seriously mentally ill people are at greater risk for violent behavior than the general population.
It doesn’t seem like we take mere threats very seriously.
Should we? That depends on the false positive rate, which cannot be determined by an investigation of shooting incidents. E.g. all sorts of internet celebrities receive tons of hate email with death threats, rape threats and what-not, but the only marginally plausible instance of such threats actually being put into effect is Spencer getting punched at that rally.
How many of them smoked? Maybe the end of smoking is making mental health issues more acute.