What? he wasn't Russian?
Yes, of course he was; but he was not a politician.
He was an Anti-Organised Crime Squad cop. He and some colleagues accused high ranking politicians of ordering the assassination of a crime syndicate boss, probably to protect their own skins.
Livinenko was arrested and charged with something or other and eventually came to London and worked with British Intelligence.
He wrote a book, or maybe two, and accused various people in the cops, army, and politics of using violence to help get Putin elected.
When he died Putin was Suspect Numero Uno. And, apparently, still is.