The locked cockpit door has been implicated in a number of pilot suicides: German Wings, Egypt Air, MH370 and possibly others.
Then there's Helios that crashed near Athens. The pressurisation failed and the cockpit oxygen cylinder had been left closed. The preflight check of the crew oxygen mask flow had not been done. By the time a cabin crew member with portable oxygen figured out how to get through the door, the fuel was about to run out.
Maybe the crew member could had landed the Helios plane, but by that time everyone on board were already dead. Still, kudos to the guy for fighting tooth and nail till the very end in this impossible situation.
Then there's Helios that crashed near Athens. The pressurisation failed and the cockpit oxygen cylinder had been left closed. The preflight check of the crew oxygen mask flow had not been done. By the time a cabin crew member with portable oxygen figured out how to get through the door, the fuel was about to run out.