Surely there is a case to say that Dr's should be obliged to inform an airliner/ employer/ regulatory body when a pilot is unfit to fly...
If the Doctor had informed the airline, it probably would have prevented this tragedy. However, the other 5 major murder/suicide incidents would not have been picked up or prevented by a Doctor. And I think that is important - those 5 pilots were fit to fly, with no history of mental health problems.
However - all 5 were experiencing one or more feelings of shame, anger, resentment, frustration and rage. Triggered by work or personal situations and the perceived attack on their inner sense of self.
And I suspect the same will be true here. A trigger event and his personality profile created a deep inner rage / frustration - and led to his (and the others') desire to punish 'someone' for the intolerable injustice they imagined had been visited upon them.
As such, his medical depression may be contributory or it may be irrelevant. Either way doctor-reporting won't stop it happening again. Preventing the opportunity to be alone at the controls has worked in the USA and may turn out be the best solution of all.