I had an experience of being on an airliner when a passenger a couple rows behind me went into diabetic coma. It was an unhappy thing to see. Passenger's relative seated right by said they had some medicine but they checked it. Plane made a cordscrew descent to the nearest major airport and passenger was wheeled off. (I called the airline to see what happened and was not surprised to be fobbed off).

In that circumstance the fellow passengers and crew did all they could, but the patient had the responsibility to manage their condition, and through absence of mind or overconfidence they didn't make out so well.

Likewise in this circumstance, I heard more than once that the (alleged) wife stated that the man who was shot was bipolar and "off his medication". It's not clear at this point if the sky marshall's even were within hearing distance of her.

Tragic all round. The wife, the shooters, the victim, the witnesses.