The KGB killed him in retaliation for taking hard lines on Cuba and the Communists in SEA. It's a no brainer.
Yeah, that idea's a no-brainer.
Lee Harvey Oswald was a complete loser. Nobody with an IQ greater than a turnip's entrusts a sensitive operation to someone as messed up as Oswald was.
And Oswald acted alone. That much is easily documented by everyone except the hardcore conspiracy nuts.
Suggested reading: Case Closed by Gerald Posner.
There's a simple reason why no one wants to believe that Oswald acted alone: we don't want to believe that one deranged loser could--and actually did--kill the President.