He left the world through a window high up in his private house. Any assassin has got to get into the house and find him first which would not be easy unless a treacherous servant was involved. Ask yourself how long the Russians had been in the area, whether their commander (I don't know who he was, do you?) was likely to have had time to set up such an arrangement.
If he had been found beaten to death in the street then obviously the Russians would have to be considered but in the circumstances it is all so improbable compared to the accident or suicide theories that it is really not worth considering.