Talk about trying to close the door after the horse has bolted. An aircraft full of passengers and aviation fuel is no place for a firearm of any sort.
The problem has to be addressed on the ground before the passengers board the aircraft, through additional security screening. This is not an easy option, as it implies, additional inconenience for passengers, and higher levels of professionalism and accountablity for airport and airline administration and front line staff. The consequences of not implementing these checks are already evident.
Something needs to be done but arming the cockpit crew is not the answer.