Slight difference between heavily armed cops with lots of backup and disarmed passengers trapped on a plane with a lunatic.
There is a slight difference, you are correct. However the attitude that you displayed (perhaps in jest) in your post that "anything I can think of that
could happen is a credible threat and can be defended against." is very much the attitude that has been adopted by some LEO's and leads to double tapping weeping folks that are crawling on the floor.
Not everything that could be a threat is a credible threat. Context matters. A nic fitting, aggressive lady on an airplane might need to be restrained, but almost certainly does not need to be beaten into submission. "I'll kill everyone on this plane", absent some evidence that she actually could, is not reason to pretend she's threatening the lives of everyone on the plane.
I would also posit, perhaps only a little in jest, that given the amount of grouches, curmudgeons, and plain aholes on this board, perhaps we should not be quick to espouse that everyone who is disruptive and says intemperate things needs to be "tuned up".