It certainly fits with KGB operations in the past. Russian agents love nerve poisons like Sarin and radioactive polonium injections for their “wet work.”
Your CIA theory has a massive hole in it: Putin actually threatened to kill the double agent, before the traitor kicked the bucket.
Secondly, the CIA had no reason to kill Sergei Skripal: he had betrayed Putin and was cooperating with the West.
Why you believe your own Government is guilty of murdering an ally when Putin has already declared that he would have Skripal killed as a traitor is beyond me. Time to drop this particular Conspiracy theory.