It’s purported by some studies, not without a lot of controversy, that mentally ill people are more violent than neurotypicals.

Pretty well established that high levels of stress and subsequent PTSD tend to lead towards shorter tempers and more violent behaviors. Similarly, malnutrition heightens aggression (ask literally anyone with low blood sugar issues what it does to their attitudes) and sleep deprivation heightens aggression and physical discomforts from extreme temperatures or chronic pain heighten aggression. All of this is correlated with mental illness.

These are all environmental factors, though. If you don't like dealing with people who are sleep deprived, malnourished, in chronic pain, and under enormous amounts of stress... there is an abundance of social policy at hand to fix all of these problems. It all just costs material resources and human labor. And, in America, that means it costs money. So we don't do it.