I have to ask how did this obviously deranged individual get the gun?
He had no criminal background or convictions, why not.
Now he was determined by the Military to be unfit, but they did not elaborate.
Every human being has the capability to do such a thing out of anger, revenge, or just stupidity, most do not act on it, why is it only then we consider a person deranged.
The military could have or should have shared that information with law enforcement which in turn would probably brought attention to him sooner, who knows.