You wrote, ". . . there is absolutely no evidence that anyone did murder him"
He left this world via a window.
"- no point in looking for a motive until you are sure there is a crime! "
Having a large group of professional killers close by, filled with hate and feelings of revenge against N., is a lot of motivation to just ignore.
"It would appear Berthier was generally liked and respected by his opponents, I can't imagine any of them having a serious grudge against him."
Berthier being part of the army that invaded Russia would certainly have been enough motive for many a Russian soldier to have just a bit of a grudge.
--Review the exploits of Russian col. Figner in 1813. He vowed to execute every French soldier that came into his hands. Figner was not an isolated case. The feelings of hatred against N. and the French Army ran deep and dark in the Russian Army.-
Also, no grudge was needed for his assassination. Eliminating Berthier just makes strategic sense. Berthier was a key person for N. It's been written that the lost of Berthier was a large blow to N., especially since his position had to be filled by Soult.
Imagine the humiliation that would have rained down on these "honorable" Russian soldiers if Berthier managed to escape from them and then reach France and joined N.