This really isn't a fair question... in fact, we need to think about the reverse.
Most mentally ill people aren't a threat to others. In fact, mentally ill are far more likely to be victims than anything. I've known many mentally ill people, including two people with untreated schizophrenia, and I've yet to see any of them cause harm to others. I personally have been diagnosed with mental illness, and the primary cause of my own illnesses is abuse. The mentally ill are far more likely to come to harm than to cause it.