More plausible? Current Russian government wins less than anyone in this political debacle, less than literally anyone. They literally have no reason to do that.
And before people actually start this whole "he was government critic" - he wasn't he supported almost everything that United Russia spew out. He also initiated campaign against Pokemon Go in Russia, he also lied about fighting in Afghanistan, he also known for selling
someone else's property, he is also known for lying about his vote in Duma (you know, this whole "he is a government critic" started from his lie about him "not voting "yes" for accepting Crimean referendum and making it part of Russia (which is understandable lie since he planned to get Ukrainian citizenship), but the only one who voted "no" for this was
this guy.
So, the dead Russian politician is just that, dead Russian politician on Ukrainian soil. He wasn't a threat to "Putins regime" or whatever people call it, Russian government was least interested in killing its own ex-deputies (at least this specific one)
FSB do not operate in other countries like that afaik, he did not had any FSB guard in Kiev, because there is no FSB operatives in Kiev.