(Not an aviation or medical professional.)
Have I missed something? I've seen no mention of this guy's earlier, pre-flying background. School? What the other kids thought? Home? Social status?
The depression diagnosis might be irrelevant/wrong - like flu and Ebola have similar symptoms. The mental state to commit mass murder (without political motive, where you are killing the enemy) seems so uncommon that the medical profession may not have enough data to predict it.