It is not hypothetical!
It was a while ago, but there was an ATR crash in Morocco when the male Captain did an intentional CFIT with the female FO unable to overpower him. The CVR told the tale.
There was that Egyptair crash out of New York and a crash in the far East also where there was fairly convincing evidence that the PF, alone in the cockpit at first at least, caused the crash.
There was a PSA BAE 146 crash where a cockpit intruder shot the crew and crashed the aircraft, and a FedEx near-crash where an intruder attacked and severely wounded the crew who managed to make a safe landing.
If you add a gun already in the cockpit to the mix it can be seen that you are increasing the level of threat that is already present.
All you have to do is to think about what we already know about the reaction time. You need to perceive the problem, come up with a reaction to the problem, and then react to the problem, when all of this can take five seconds or so, a very long time. If the person at the controls, the PF, were to do something completely unexpected, the PNF might not have enough time to understand the new, completely unforeseen, circumstances and then come up with an appropriate solution.
In this case the Captain went a bit funny in the head at cruise altitude. You don't want to think about what might have happened if he had saved that up for a critical phase of flight, or quietly waited for the FO to step out for a break, or pulled out his TSA-approved pistol and told him that Jesus was now guiding the flight. These are all examples of 'thinking the unthinkable,' by definition. In the same way, up until 9/11 the idea of hijackers who were suicidal was also unthinkable. Now we think about that, and we are going to have to think about this.
We are again moving into the realm of the hypothetical from the realm of the unthinkable. Three Mile Island and Chernobyl were like this; it was unthinkable that a gang of screw-ups could be running a nuclear power plant so that it was not really held to be hypothetical.
There was a movie by Jim Jarmusch titled Down by Law, where one of the characters spoke of his Italian mother killing a rabbit to cook it and eat it, how she would hold the bunny stroking its fur, so nice, and then with one sharp blow she would kill it! The scene worked because of the juxtaposition of the loving mother being nice to the nice furry bunny, all nicey-nicey, but then Whack! Well, 'true to life,' I guess you could call that scene. So now we have the senior Captain, a check airman, flipping out. Two weeks ago it was a sick fantasy, but now it's true to life. We are just waiting to find out 'Why?'