Harris and Klebold were mentally ill and should never have been allowed in a public school. They should have been locked up permanently the minute they turned 12.
Not at all. I have followed that case closely and read several books on it. There did not seem anything wrong with them until close to the event. There was a little unimportant crime, but there is a lot of that in high schools.
As time progressed closer to the shooting, Harris probably did develop schizophrenia and Klebold became seriously depressed. Klebold did NOT show it, however --- he got into college, got ready for college, went to the prom ---- it was impossible to tell the shooting might happen.
Young males can develop schizophrenia very fast around late teens. A lot of shootings are a result of schizophrenia that no one identified. I don't know what we can do about that, frankly.