Old Fella
My thoughts exactly - the combination of a flight sim available to practise the evasion, a willing trainee who simply would do as he was told (get me a bottle of water from the galley) and the mechanical deadlock in the door handle would give the Captain all the time he needed to carry this out. Single user of the flight deck O2 system gives many hours of continuous use allowing depressurisation and thus nobody conscious except him.
The question remains why?