You wrote, "Neither the Bavarian police - or better police of Bamberg - nor the Russians had any motivation to assasinate Berthier - what would be the reason?"
The Russians had no motivation to assassinate Berthier? Only a closed mind could make such a statement. Do you think that by 1815 that the Russians would have forgiven and forgotten what happened in 1812? You must be thinking about a much more peaceful and pacific type of Russian soldier than I have ever been made aware of.
It was war! Napoleon, the very man who defiled Mother Russia, was again loose to bring more death and destruction to the world! Berthier was well known as N.'s right hand man. It was known that he was anxious to return to France. What more motivation would be needed?
Of course the Russians were polite and respectful to Berthier but that doesn't mean a thing. Who is to say that the Russian officers who met with Berthier were the same who assassinated him? It could just as easily been another group.
Without question, to any right thinking Russian, Berthier was a direct threat.