There are no evidences to any theory, so Occam's Razor is on the side of the simpliest theory. And the simpliest explanation, without any unnecessary elements (like mythical Russian murders) - is the leak from PD labs, or some kind of human experiments, or serial killer, working there and so on.
Alternatively, there is also the theory that The Russian State, which has established a well known and highly publicised Modus Operandi over several decades has decided to 'do' yet another one.
And when you add a Russian Spy/Defector into the mix and you apply Logic and insert The Balance of Probabilties then you could be forgiven for thinking the Russians probably did it.
For the hard of thinking who cannot understand the forensic principles of Logic, Modus Operandi and Balance of Probablities I give you the following simplified exemplar;
The Duck Test.
It if looks like a duck,
Walks like a duck,
Quacks like a duck,
Then it is probably a duck.
Duck test - Wikipedia
There is also the Elephant Test* which can be applied alongside the Duck Test and indeed both tests would appear the fit the Salisbury incidents.
*Which is about elephants in rooms and those who pretend not to see them, an allegory.