So far, everything on here is pure speculation. The only things we can take as reliable information is from the people involved officially in the investigation. They have so far provided us details of what they believe happened with the evidence they have so far. This is their expertise so we can be confident that they have analysed this evidenence very carefully to be able to be confident enough to release it to the public.
There is clearly more investigation to be done and I'm sure this information will become public when they are confident to give it.
At the moment all we know is the F/O manually changed the altitude selection to something lower than 38000Ft (unofficially it seems 100ft) and allowed the aircraft to descend on autopilot. Also that he was unresponsive to the Captain trying to gain access to the cockpit and also unresponsive to ATC. It is not yet been confirmed whether the F/O moved the toggle switch for the door to lock or whether the Captain failed to enter the code.
We know the F/O had previously suffered depression in his career and we now also know that he was receiving treatment for something undisclosed at the moment.
I like to see things by facts and not speculate. I'm sure that it won't be long before we find out exactly what happened and hopefully the investigation may produce findings to stop something like this happening again.