The purpose is to demonise Russia.

No need to demonize Russia, Putin does an admirable job of it himself.

A short list of Putin's victims:

Sergei Yushenkov
Paul Klebnikov
Alexander Litvinenko
Anna Politkovskaya
Natalia Estemirova
Sergei Magnitsky
Stanislav Markelov and Anastasia Baburova
Boris Berezovsky
Boris Nemtsov

These are just the most prominent victims, there are others like Vladimir Kara-Murza who survived two poisoning attempts and several who "accidentally" fell out of windows in tall buildings or suffered unexplained "heart attacks" while in jail.