Not particularly. Just because something CAN happen, does not mean it is remotely likely to. For the mentally unstable, there are far more simple and effective means to get that forever sleep, and for those that would plan something like this out, well short of going full blown paranoia mode, not much would stop it. Heck, even if FFDO was extended to these ops, good luck drawing, accurately hitting, and stopping a determined , “unarmed” threat (which takes many more shots on average than one thinks), while strapped into your seat with limited mobility….odds are even worse if you're SP.
Admittadely that was a largely anecdotal answer, but think for instance about reaction times for a pilot to a normal mechanical, no b/s emergency….now imagine that emergency is your passenger, who is actively fighting you (or at least your airplane). FedEx took the coordinated efforts of three highly experienced pilots to save that ship, and that would have been virtually impossible if the 2-1 advantage the pilots needed while subduing the assailant required that the plane be left without someone at the controls.
Thankfully, I’m primarily a freight guy, but if a person jumps out of box on me, chances are his wishes will be realized…unless he is slow and my crash axe hasn't mysteriously jumped out of its bracket again.