But in the brief moment when the first officer was waiting to enter the lavatory and the crew member to enter the cockpit, the captain locked the door, becoming the only person on the flight deck.

While the exact nature of the disagreement between the two pilots is not known, the captain was clearly rattled enough to take this extremely unprofessional step to lock his co-pilot out. According to EconomyNext, senior cabin crew members had to persuade the captain via the communication link to let the first officer in

That should be prosecuted just as any disorderly passenger is. Maliciously violating safety protocols in such an egregious way is distracting himself and the rest of the crew to risk the lives of everyone. He held the plane hostage.