Just because someone has no history of obvious mental illness does not give that person a clean bill of health. Can you always distinguish someone with mental illness from someone who is "normal"? No, you cannot.
Mental illness can manifest itself, overtly, in many ways. And sometimes in ugly ways.
Airbubba:
I found your comment about dietary supplements, such as those the Captain was supposedly involved in, interesting. And especially after watching a video of one of the passengers - ex NYPD I think - who was sitting in row 10C. He claims that there was a period of 15-20 minutes before he helped subdue the captain in which he observed the captain drinking copious amounts of water and exhibiting signs of "cotton mouth". I'm starting to wonder if perhaps the captain had imbibed in a bit too much of the wares he was trying to market? Who knows what the contents are of this dietary aid?