The Captain was obviously acting strangely - the report indicates the crew were thrown by his sudden change of behaviour so presumably he was OK arriving at Sochi and if they suspected he was drunk they wouldn't have been surprised....

I wonder if it was something like diabetic shock (hypoglycemia) or a stroke.....

As I told previously and as can be quite clear read from this four leaked pages, crew (not only this one but whole Syrian Express) was under constant pressure and workload unusual for them from previous years.
Crew has a proper rest hours before this flight, but whether they spent them sleeping in their beds or running around preparing family celebration of the upcoming New Year eve?

This is another (very possible) point of view. If PIC was behaving stangely could be because of a lot of reasons.
And remember: all the "flight" endured 73 seconds.
The nav and the FO were busy on their duties and the (possible) reaction to a strange behaviour on the PIC's side would come when no recovery was possible.

No alcohol or drugs were found in captain remains. I believed (based on listening CVR and ATC radio) that captain was not so emotive or confused as it can look from this discussion.

In the report, as one of possible reason for loss of spatial orientation is mentioned the "direct" system of artificial horizon. In Russia, two types of attitude indicator are used. First of them is the same as AI in western planes and it was the type in this Tu-154B. The other type has different indication - in front of sky-ground cylinder a little plane symbol is placed. This plane banks left and right - of course in opposite direction than horizon line in "western" AI.

In the case of of very stress or panic situation something in your head can switch to old reflex you got many years ago.

That sound like:
https://en.m.wikipedia.org/wiki/Aeroflot_Flight_821

I already mentioned this, some pages ago but nobody notices.

Together with the Kazan Boeing CFIT incident on a banal TOGO and the or , just to see what happens..... or

Shall we say, some Russian pilots have either a fairly shallow relationship with reality or those all got the Darwin awards? :D