This guy hadn't been in the game very long. That may be a factor.
I'm surprised this hasn't come up more. I know it's different in Europe and they probably really don't want to go into this area as it is too uncomfortable and difficult as the ramifications would be considerable. In the USA that guy would simply not be on the flight deck with their 1,500hr rule. I was still flying aircraft with 2-6 seats with Lubitz's hours. I would hazard a guess that a great many of fellow Aussie pilots would be the same.
A disconnect from reality and a complete failure to identify and understand the gravity of consequences seems to be apparent with school shootings and other cold blooded mass murder type events. When the reality of things isn't quite the same as a video game it is no wonder individuals end up pointing the gun at themselves in the end.
One does have to wonder if the road into an airline cockpit had been longer and via a more 'traditional' route whether this would have occurred? There certainly would have been more time for problems to surface and be identified before a person reaches a position of great responsibility.