The theory of a drunk PIC is a bit difficult to digest, IMHO...
The AC was making a STOP to refuel.
@Kulvestukas ensured no extra cargo and no extra pax were loaded except FUEL.
The crew was piloting from Moscow and the procedure seemed to be under regular conditions. If any (or two, or the three) were drunk, things could be difficult to make a night flight under such conditions (first leg).
If the PIC (or another crew member, or all) went "wet" during the stop at Sochi, could make it possible, but apparently the nav was on duty and acting correctly.
Some old leak also told us the FO said (in a correct manner) the PIC something like: "we're falling, commander" (this read in this thread, months ago).
So it's left the PIC alone to be drunk.
Drunk enough to make a flyable AC go south? And the other two crew members went lambishly to death knowing the PIC was intoxicated? Were there any clues in the cockpit or ATC recordings to show a displeasure behaviour, this meaning something suspicious in the PIC's voice?