You have to be realistic and not dreaming of impossible scenarios has to counteract this kind of action
There are people who commit suicide properly (they kill themself only) and others by example open the gas committing suicide and kill all the other inhabitants of the building
When a person mentally disturbed wants to kill herself she will at all times and eliminate those that would prevent her action
The second pilot shall be on the flightdeck and been neutralized by the other crazy pilot (stabbing is easy)
It was just an exeptional event ... and no rules will change because that