Back in the dark ages of the 1950's and earlier we had large state institutions for the insane. People were committed and force fed drugs to keep them complaisant. It was horrible. Of course, the pendulum swung too far in the other direction and now it is nearly impossible to get a psychiatric commitment and then they are usually only a 96 hour hold.
So now we have a whole group of people who should be institutionalized, who aren't and it hasn't worked out as bad as many thought. However, there are exceptions and when horrific crimes are committed we wonder why no one picked up on their bizarre behavior earlier. And of course, people had, but their hands were tied unless the person presented an imminent threat.
The sad truth is, that even professionals can not always or even most of the time tell with any precision when a mentally ill person will lose it and commit an atrocity. The even sadder truth is that there is so much stigma to mental illness that most people who need treatment don't get it because they can't afford to be labelled and also they can't afford the treatment.
As much as people seem to hate the Affordable Care Act, one of it's goals was to increase insurance coverage for mental health treatment. Perhaps over time, it will make a small difference.