Weapon
I would suggest that the aircraft is the weapon.....as was evidenced by the Egypt Air incident.
The only person capable of commandeering an aircraft is an existing aircrew member.
This person can either be a sleeper or someone who has gone over the edge.
Airport screening cannot detect either.
I suggest that the airlines themselves are responsible for both scenarios through self monitoring of personnel.
A CC member with a CPL is also a possibility.
All have flight deck access.
None of the terrorists involved in 911 was employee of the airlines involved.
If wrong I am sure I will be corrected.
Perhaps the screening of crew should be done on the ground and at the entrance to the aero bridge.
You are certainly more visble to the pax on your aircraft.