With the information I have read thus far, the only problem I see is his personal private physician not reporting his illness to the airline safety administration (whatever that is in Germany). But, not knowing German law, I don't know that the physician could have legally done so, if he/she can and didn't then the blame lies with them. But then again I don't know if his personal physician, assuming not a psychiatrist, was fully aware of how bad things had become. There were reports of him(the pilot) tearing up doctor's notes but we don't know what's on those notes.
I know here in Ontario, as has been mentioned many times in this thread, doctors are legally required to report to the Ministry of Transportation if a patient has a condition which may make them unfit to drive a car. This was born out of pressure from the medical community itself when doctors were having a difficult time trying to convince their patients to voluntarily not drive based on their medical conditions and were, at the time, legally forbidden from revealing any private medical information to government.
As with all things, hindsight is 20/20. If there were signs and reports of troubling matters with regards to this individual that weren't followed up according to protocol, then that's a liability issue. If there were things that could have been done but weren't because the protocols weren't in place, that's a societal issue that needs to be seriously evaluated and addressed.