Kennedy had been having an affair with a communist spy and the story was about to get out. Think about what that would have done to the Democrat party at that time. Add in all the other Kennedy debacles, and the Democrat party would have been in the can. It wasn’t just Johnson, though he had the most to gain from it. It was the Democrat party.
We know it wasn’t the Soviets, who regarded Kennedy as a patsy when it came to arms treaties & therefore valuable to them, & they would have kept Castro on a short leash for the same reason.
Hope Robert A. Caro lives long enough to finish his multivolume opus on Lyndon Johnson. Just how evil LBJ really was is only now being realized.
The dem party in general does not appear to have been in on the assasination and when you think about it, if any more than a half dozen or so people had ever known anything about it, the whole thing wouldn’t have worked.