Mental illness as a culprit is a pretty tired explanation of these events. Statistically, mentally ill people are far more likely to be victims of violence than to perpetrate it, and in many of these events, there is a clear lack of diagnosable or observable illness to begin with. Besides that, even if you don't believe the numbers here, it's possible to improve mental illness treatment and legislate gun control responsibly, it's not an either-or thing. TLDR "Mental illness" is a very tired, very poor explanation for the phenomenon, not even to mention it perpetuates and encourages stigma.