Mental illness is not neccessarily the cause of anger and violence. What mental illnesses can do is overwhelm, or prevent individuals from learning, the inhibitions they need to function properly in our society; therefore, certain mental illnesses make it more likely individuals will respond violently in inappropriate situations and in inappropriate ways. Behavioral modifiers like drugs, alcohol, extreme stress, and toxic relationship can also play just as significant of a role in violent crime as certain mental illnesses.
Because mental illnesses include a broad range of conditions that impact the functionality of people to varying degrees, the impulse to simply blame mental illness when a mass killing occurs stigmatizes those suffering from mental illness. In turn, this makes already reluctant individuals suffering from mental illness more hesitate to address their issues while it also deprives them of the social support structure they need to face their demons.
At the same time, it is important to remember the sole reason for discussing the cause of a crime is to help prevent future crimes. Framing violent acts as a result of mental illness prevents our society from properly addressing the factors that lead to violent acts. The truth is that anger and violence are part of human nature, thus these elements of our personality need to be recognized and controlled. As a society, we need people to address their emotions and impulses in healthy ways that do not undermine social cohesion. Consequently, our discussion of mental illness when it comes to violent crimes must focus on mental illness resulted in a particular criminal act and what solutions might prevent particular situations from arising.