I can assure you the people, who should know the code, don't necessarily know the code. Many airlines, routinely leave the cockpit single crew, when another pilot take a break or a comfort stop.
The second point I made, was the code may have been changed as part of a company specific security protocol and not propagated to the crew via communication or in the tech log. Due to familiarity/aircraft change or rushing, the code not confirmed or checked pre departure. All these elements have human factors related threats.
Sensible airlines never leave the cockpit single crew. This is not an industry wide procedure though.
There is also a huge difference between suicide and mass murder and torture. Sorry but these pilot profiles, don't point towards this.