Is there the slightest evidence he was, other than 'some other guy in Russia was murdered with polonium once'?

You already know that the person who claims an extremely rare event occurred is the person who has to show up with the evidence. Without any evidence, why go around making this sort of inflammatory comment about a very sad event?

Because it is Russia and no getting killed in Russia over politics or business isn't a rare event.

Plus, that guy wasn't even in Russia.