Best way to act with a "situation" developing in the cabin...
Don your oxygen mask.
Pull all the circuit breakers for cabin oxygen.
Depressurise.
Unless the hijackers are equipped with portable oxygen, their time of useful consciousness at 35,000' would be pretty minimal.
As regards locking the flight deck door, I'm not willing to cross my legs for the duration of the flight... and how are we supposed to get cups of tea? Install a cappucino machine in the flight deck, methinks...