That's true, mental illness is a broad term that does encompass both functional and seriously impaired individuals. The fact of the matter though, is that this individual was deeply disturbed and it likely played a heavy role in what happened yesterday.

Yes, many people with mental illnesses both mild and severe make it through the day, year, and even their entire life without doing bad things to people. But others don't have good support systems, proper medical care, or any capacity to control their disease. People in situations like that are much more vulnerable to losing touch with reality and the ethics and morals they would otherwise have.