Facts
Still, today, there are posts questioning how the Captain was unable to gain re-entry to the cockpit. Please can all posters read some pages back to discover that the re-entry has to be either authorised by the remaining pilot, or can also be denied by them. (the possible use of the emergency entry code can also be denied).
Almost immediately after the Captain left the cockpit, after announcing (heard on the CVR) the co-pilot is in charge, the co-pilot dialed up altitude "zero" and the plane began its descent. There is only one conclusion people. Further to this is circumstantial evidence, leaked, that he used suggestion to go to the toilet to encourage the Captain to go at that time. Assumption is knowing the Alps were in front and descent would have a very predictable end at his favourite place on the earth.
On the CVR and because of the pounding on the door, we can deduce the denial of re-entry of the Captain to the cockpit. Go ahead - tell us that's because the co-pilot was frightened or something. That's ACTIVE denial folks - using the toggle switch to lock the door. Timeout whatever. He could have also used the toggle switch to unlock the door at any time. He didn't.
All this while the plane descended, which we can assume the Captain and flight crew understood, as well as a growing number of passengers who eventually ended up screaming. If not all of them. Horriffic. Let's not be in denial of that. Hard facts people.
So please don't be in denial of the facts. The co-pilot locked out the Captain and descended the plane into the Alps. Cannot be clearer than that. He could have admitted the Captain at any time. He didn't.
What reason could the co-pilot have for not readmitting the Captain if the plane was in trouble? If the co-pilot was incapacitated, how could he actively deny entry (that's what it takes). He was not incapacitated, in fact he probably denied entry to the Captain more than once in the 8min descent into the mountains.
Get real. The co-pilot on this flight crashed the plane on purpose. Now - deal with it. Why - is not so easy to answer.
I like the French investigation comment. Suicide is taking your own life. Crashing 150 people on a plane into the French Alps - that is mass murder. That is what we are dealing with. The guy is a monster. For whatever reason.
Depression or not, what we are dealing with is mass murder. Has happened before as well. Not easy to answer why, or how to prevent it.
Stop looking for excuses for him. It is indisputable. It's on the CVR. Maybe there are more like him. That's why two people need to be in the cockpit. That doesn't mean it cannot happen again, or some other way, but it sure lengthens the odds people.