it's totally unacceptable
To cause somebody to die a slow agonising death, which poisoning often is, is unacceptable.
Not all of the Russian soldiers are there voluntarily. But either way, there are conventions about what is legal and what isn't and it's not legal to poison other people.
If they do it, they won't be found out and will not be penalised. The
Russians will be held account for war crimes where possible.