A conspiracy of that magnitude would have eventually come to light. I do not believe in conspiracies lasting for a long time and involving many people. Someone will leak information to the press, get drunk and brag to his wife or girl friend, or have a death bed confession.The CIA killed both Kennedy brothers.
They may have used Oswald & Sirhan to pull the triggers but they were pulling the strings
People use conspiracy theories to derogate people they already dislike.
Lee Harvey Oswald was a Communist sympathizer. He belonged to the Fair Play for Cuba Committee. He defected to the Soviet Union. Nevertheless, conspiracy buffs do not blame the Communists; they blame right wing hate targets.