I disagree whether that guy Cho was considered mentally ill. 20 years or more ago, he may have been asked to leave the school. Institutions can longer do that with people they feel may be dangerous or even suspected of mental illness. They would be sued under the ADA. Crazy and potentially dangerous people have "rights". In recent years their rights seem to trump the rights of society to be safe.

Cho was a classic failure of the system and of the psychological industry itself. He was mentally ill. His classmates knew it. Most of his teachers knew it. A district court had declared him mentally ill and a danger to others after some incidents such as stalking.

He was declared mentally ill by a court so you can't say no one knew Cho wasn't crazy.

The system fails to protect society as a whole. Privacy and anti-discrimination laws keep people like Cho in schools and in workplaces.

You haven't noticed the severe uptick in things like workplace and school shootings since these types of special interest laws were put into place.