The mere presence of another human, with eyes and a brain and a moral conscience (as opposed to a passive video system) changes the psychological dynamics in the cabin, compared to being alone. There are some things it is simply much harder to do - psychologically - when someone else is present and observing you.
Think not? Why did this co-pilot wait until he was alone to take this action? Egypt Air shows that he could have - with some probability - brought the aircraft down even with the PIC trying to stop him in a full fist-fight.
But doing it alone - in "secret" and hidden from the eyes and judgements of others - made it easier.
All very valid points! lurch into the mind of an narcissist or sociopath and you will find they are wimps, pretenders and cardsharpers (and sometimes pilots). Social control is what keeps them from doing what they wish to do in their mind. For me it looks all very considered with the information available.
Destroyed sick leave note and then a cool and normal flight until the door closes - from the outside.
What is he doing to his family? his friends? his colleagues ?
Looking at the facts already available, the deeply pathological narcissistic actions of this chap are so utterly embarrassing. He did not only kill 149 people, he destroyed the live of many others, but these people are not dead, they have to get on in live. What damage and a name in history because he was obviously not getting in his live were he wanted to get to.
I really don't think the real issue is "depression", it seems more like a well hidden serious personality disorder.