Here you go Steve:
https://mwhealth.org/stories/policy-watch-mass-shootings-mental-illness-what-the-research-tells-us
Yes, someone who commits mass murder is clearly emotionally unstable, but a study by the FBI's Behavioral Analysis Unit of 63 active shooters found that only a quarter of those had a diagnosis of mental illness. Other studies have found that mass shooters are not acting out of impulse but have deep grievances and anger that drive their actions.