All the sequence of events would have been set in motion, just due to one moment of madness by the co-pilot. Might have been depressed or what, might have engaged in fantasies about doing it. Pilot left, may be a restroom break, doors locked automatically, comes back and knocks to be let in. At that moment, if the copilot has decided to act out his fantasy for even a few seconds, he would not be able to reverse course easily. He has to explain the delay, there will be investigations, skeletons in his closet might come tumbling out, at that point co pilot would be in total panic mode with his whole life crashing around him. He would be totally irrational, 38000 feet, 4000 feet per min, it took the plane 10 minutes to crash. Passengers would be in panic, the locked out pilot too. Very very tragic.
Surprised USA has a policy of not allowing the cockpit to be occupied by a lone occupant. Given the staffing cuts and the callousness of the airline management surprised they did not mount a lobbying effort "against the onerous and burdensome regulations by Washington bureaucrats that is strangling the industry and killing jobs" to remove that requirement.