I read some of his manifesto and it looks to me from that that he had been very disturbed for a long time, extremely paranoid, and grandiose as a defence against his deep feelings of humiliation. It seems to me that he was clearly very unwell.
I really don't see how not talking about his mental health issues is of any help to anyone's understanding. He may not have fitted any diagnostic criteria - so what? - I see kids who don't fit psychiatric classifications who are clearly disturbed and will grow to be more so, acting out their fantasies as they get older, if the therapeutic work they're engaged in isn't helpful enough. More honesty about the levels of disturbance children can show and the long term intensive work needed to help them is what's needed, not not talking about it because of the stigma involved in psychiatric diagnoses.