Here's the motive if you want to go this way. Giancano got fed up after Bobby had Marcello grabbed off the street in New Orleans and released in the mountains of, I think, Guatamala without wallet or money.
Giancano sent Johhny Roselli (the same guy who was going to deliver the poison gigars to Castro)and another guy to the Kennedy compound in late 1962 to ask Joe to talk to the boys about who got them into office.
Joe said he would, when the family was together at Christmas. Before then, Joe had the bad stroke and supposedly the discussion never took place.