As someone already told you the other day, the Soviets just liked to kill people. Look up the tragic story of Colonel Kuklinski, a traitor to the Warsaw pact, whose family members were systematically murdered in the West, long after he spilled what he had to spill to the CIA.
But they never did it senselessly, without reason. In the case of Col. Kuklinski, the case was effectively made for other potential defectors.
The Soviets were calculating and they were intelligent. They didn't act without regard for their own best interests. And assassinating an American president was fraught with great risk...but without any meaningful benefit.
Further, for purposes of argument, let's stipulate they DID do it. In which case, they never would have involved Oswald -- with his direct connection to the USSR. They weren't stupid.