In this case, the argument isn't so much about Russia being concerned about getting caught. The "message" being sent is to their own operatives, if you turn traitor and flee to another country, we're going to get you(and maybe your family members too).
But that's not what happened. The guy was arrested and sentenced to prison in Russia, and was subsequently pardoned and shipped to the UK as part of an exchange program. So now, 8 years after being in the UK, he's assassinated. Yes, he was a former spy or possibly a double agent, but even so - why now? Surely there was a Russian election between 2010 and now, so why this timing? The "message" you suggest Russia might be sending is just as conveniently timed as some other party knowing this and timing it so that it would look like Russia did it right before an election. I see no way to distinguish between the two cases, and this non-falsifiability is exactly the point. All fingers will predictably point to Putin and there's no way on Earth Russia or anyone else could demonstrate otherwise.
Although realistically, in that respect, the "more likely" option here is Russian criminal elements showing their hand outside of state direction. They potentially have access to that stuff as well, and they're also not care if it reflects back on Russia, as the government will take the heat with only token efforts to track them down, so no risk to them. At least so long as they limit their targets to "Russian traitors."
That's the trick here. The West doesn't care what actually happened, so long as they're satisfied to have more proof that Russia is evil and behind all problems. Once the media tried, for instance, to blame the instability in Syria on Russian interference, it should be clear as day to everyone that these narratives have no credibility. So now you're in deep with the Double Cross system: do you disbelieve everything knowing that you could be fooled by any piece of "evidence", or do you just go ahead and believe the first suggestion made, for instance here that "Russia did it". No one will be looking to find alternatives, such as Russia terrorists, or private interests, or whoever else, and so the "Putin bad" narrative will go uncontested. And the sad fact is that
Putin is bad, which makes the narrative all the more prone to going uncontested.
And if there is significant diplomatic fallout for it as a consequence, well.. That'll probably just make for a banner year on the black market. Even war isn't necessarily a bad thing in their book, still great for the black market, and as they're "not the government" they'll still expect to be there win or lose on the Russian side.
This type of analysis will not be made by most people and by most governments. They are enthused by the idea of having more fodder for the anti-Russia narrative that considerable effort would be made to
ignore contrary evidence, no less look for alternative explanations when the matter isn't entirely clear.
I don't think people take these matters nearly seriously enough or are aware anymore of what it is to threaten a major nuclear power. When the matter on the table is threats against Russia people talk about it like it's a game of Axis and Allies or something. "Oh yeah, we shouldn't let them get away with it!" Easy to say when you're not the one who'd have to walk over there, gun in hand, to do something about it yourself. Would people say things so easily if the consequence was their own children going out to die the next day? That is how serious it is to jump to these conclusions. Actually it's worse, when nuclear war is a possible consequence. When there are parties out there who
want escalation with Russia so much care has to be taken to fall for easy bait to rattle sabres.
