If people want to be angry about something they should be angry at the people who let this freak out on the street knowing he had mental issues and chose not to do anything about it,
I understand and share you anger and frustration.
In fact the mentally ill are actually as a group less violent than the normal population (statistically). I this not just that they are mentally ill. It has more to do with the level of emotional stress they are under, their sense of disconnection from others, lack of coping mechanisms and skills, lack of support structures like friends, church, social peers to help them process their emotions and stress. Add in drug habits, money problems, stupid role models in our culture (TV, internet).
What we look for is patterns of inappropriate aggressive reactions to smaller frustrations, willingness to intimidate others to get their way. lack of impulse control. Fear reaction by those around them. Denial by those around them.