Mohammed seals himself in a crate with a knife or gun ... at altitude, he breaks out of the crate, goes to the cockpit, takes control, and flies into a building.
No big deal.
How is he going to get from the cargo hold to the passenger compartment? I believe they are separate compartments without a door between them. I'm sure there are experts that know more here. I was under the impression that these areas were at different temperatures and pressures as well.
I would be more concerned with somebody shipping a bomb as freight. I have heard that little of the freight that is also in the cargo hold is inspected. I can vouch for this on at least one instance. I shipped some hardware on a Saturday morning this past March using airline freight for a customer that needed it ASAP. They gave me their airline freight account number and the shipment was accepted without question, weighed (about 60 lbs in 3 boxes) and placed into a shipping container - the kind shapped to go into a commercial airplane. I do not know if there was further inspection but it did not look like that would be the case.
Better yet, Mo seals himself in a crate with a knife or a gun. The crate, which is a large container that has not been declared as needing special heating attention, goes into the aft cargo hold. The captain tells the first officer to turn off the bleed air to the aft cargo hold to conserve fuel as the airplane climbs to FL 350 (35,000 ft). Later as the crate is unloaded, popsicle Mo thumps through the wood as an ice man, his cold dead fingers wrapped around his weapon.
Yeah, but he didn't do that. I understand it is a big deal that it was allowed to happen, but they don't need to put this guy in jail.