A passenger having a panic attack is ill by definition. They dont want to have a panic attack and its largely involuntary.
For a passenger who is on the aircraft when it is taxiing and wants to take another course and isnt ill by any definition its too late. The process has started of getting him from his point of embarkation to his destination. Imagine a passenger pointing out a city in the distance during flight and saying - " I want to land there, its closer to my home" .............If the passenger wants to make a fuss then its they who endanger the aircraft and should be dealt with as such. I can recall several passengers on the same flight as me whose behaviour was disgraceful. In the average public house it would have resulted in them getting a good slapping but because its on an aircraft we depend on the cabin crew to deal with it.