The Soviets had no motive; nothing to gain from the assassination of JFK.
Indeed, as it turned out, they didn't profit from the event.
Castro, though, did have a a motive.
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.