I'm not buying the whole "mental illness". "Mental illness" could be anything. It could be that he was suffering from bulimia. I'm not going to degrade whatever his illness was , but at the same time, I'm not going to pardon his actions simply based on a broad statement of a "mental disorder". I heard that it was planned and I'm guessing that his mental illness did not prevent him from knowing good from bad in this scenario.
Who gives a fuck? He's dead. He won already. Judge all you want, or don't, doesn't matter. You and everyone else is completely powerless to do absolutely anything at all about this. Completely powerless. That's the beauty of fetishising violent revenge fantasies in a society, basically anyone can live the dream so long as they kill themselves in the end. This guy nailed it. He'll be incredibly famous, got the attention he clearly craved and will pay absolutely no cost, suffer no judgement from society. Just the way we sell it to the kids. He had a particular set of skills that apparently included shooting children in the face. That made him a nightmare for people like us.