It could be the Russians, they are more than capable of these murders. It could be the UK Security Services working to make life difficult for the Russians who have swamped London with criminal riches. MI6 is quite capable of arranging for such a show. It could be the CIA playing dirty tricks as a way of getting to the Russians as the White House seems to have been bought up by Putin.
All are possible. Which you choose, or which variety of conspiracy you vote for, is largely up to your existing perception of the world of dirty politics. It isn’t even up to wondering which is the more likely as, going on past experience, they are all as likely as each other.
As Putin is no more than a Cossack crook I would go for the Russians having a vendetta against old friends. But then there is a whole nest of thoughts about how other countries will play this fact to their advantage. That’s the trouble with dangerous games, they get more and more complex as time goes on.