Depression is a mental illness. This should have put a big question mark over his suitability to fly a commercial or any other type of aeroplane. It would seem to me that Lufthansa/German Wings are culpable. They knew he had been ill and yet allowed him to take charge of a Commercial Aircraft and the lives of 150 people.
While that's an entirely understandable point of view, such a hard line probably increases the problem.
Imagine having spent so much on training, the pressure not to report depression or mental illness is likely to be immense. It is an instant end to your career when in all likelihood you either believe, or it might even be entirely possible, for it to be controlled.
It will be a costly, but probably far more effective and cheaper, in the long run to have more permissive sick leave allowances and more open reporting of mental health issues with an accepting culture.