Narcissists aren't known for suicidal acts. He may have been some form of sociopath but I doubt it was NPD.
His actions seem very narcistic. He did not care about others at all. He choose the best opportunity to take as much people with him as he possibly ever could.
And yes, narcissists do kill themselves from time to time: if they loose the mirror to look into, the mirror feeding their insecure ego. It looks like he was going to loose this mirror in due course: his medical and then the ATPL licensense, all he wanted to become: an airline pilot. Statement by the private flying club were he became a gliderpilot.
I used to believe he had some psychotic episode but of course, what do I know. But there are facts which give a picture IMHO. What is frightening that he apparently was breathing calmly, right to the end, sitting in his seat with 149 innocent and absolutely helpless people behind him. Sorry if I am harsh and a bit direct.
I never heard anything bad from Lufthansa Pilots when talking about their company, just every pilot in Germany wants to work for LH as they have the best employee schemes. Though not as good for young pilots these days as it use to be.