"Some people with depression will consider taking their own lives, some actually do. This man may or may not have been a depressive but he was something else far worse to consider and execute a form of mass destruction".
This is a long thread and as I'm of a similar age to Funfly, I feel life may be too short to wade through it all - apologies if this has been raised before.
I agree, it seems probable that something more than "simple" depression applies here. I would have thought that had this fellow's clinical condition been less profoundly disturbed, it would have been both simple and convenient, since he was a member of the local flying club, to leave a note, book a spamcan for an hour and quietly disappear into the boonies. To deliberately kill all the others suggests a psychotic condition which would have been difficult to conceal, particularly in the intimacy of a flightdeck.
An obvious conclusion to be drawn from this is that some amendment to patient/doctor confidentiality protocols is required to allow a doctor to alert appropriate authorities of his/her concerns in respect of mental stability. Perhaps this already exists but if so, something went horribly wrong on this occasion.