Do you dispute that Russia poisoned Litvinenko as well? Enemies of the Russian state just seem to be poisoned by incredibly rare substances every now and again.
I don't know much about that case, but a cursory read on Wikipedia (which could itself be biased and/or misinformed) is a bit more convincing than the Skripal case.
Nonetheless we're getting pretty far afield. Select two incidents where any nation has killed its double-agents or whistleblowers (and I doubt there are any nations that lavish fond treatment on their double-agents in particular), and you now have evidence against that country equivalent to that upon which you declared Russia the only possible country brazen enough to carry out these two embassy attacks (assuming they were attacks).
The person I'm conversing with mentioned two, and based on that, concluded Russia is the #1 suspect here, and the only country brazen enough to do it. All you have to do is pick two assassinations by any other country, and that country is now equally brazen. The point is that the statement is weak, not that there have been two assassinations in history. Get it now?
Nonetheless we're getting pretty far afield. Select two incidents where any nation has killed its double-agents or whistleblowers (and I doubt there are any nations that lavish fond treatment on their double-agents in particular), and you now have evidence against that country equivalent to that upon which you declared Russia the only possible country brazen enough to carry out these two embassy attacks (assuming they were attacks).