... since they're never going to stand a fair trial, the stakes are too high here. As for proof, we all know the spying agencies are transparent and honest organizations. On both sides.
Yes, it is very likely that individuals from Russia are responsible for committing this kind of crimes, however the effort to connect them with the Russian government seems questionable to me, it has to serve a purpose. Makes me remember the famous episode of WMD as a premise of invading Iraq. Everybody and his dog was rushing to pile up credible evidence going up to the famous accurate satellite photographs. I know, I know, this time Western governments have proofs beyond doubt.
I wonder who wants a war with Russia and what would be the purpose. There must be some sort of a resource (no, not oil) important enough for the Western world to try to secure it. At any cost.