RFK planned to remove the CIA's clandestine services
A week before RFK was killed, he declared that he would remove the clandestine services from the CIA. Not only that, but RFK's first assumption was that the CIA had killed his brother and he posed that very question publicly.
Due to his popularity, RFK was about to be put in a position to become President and therefore actually make reforms to the CIA or even dismantle it just as JFK before him had sought to do.
According to RFK JR, Cesar was a CIA asset. If this was truly the case, and Sirhan was simply a MK Ultra patsy, then now you know why they would have a motive to implement this plan.
RFK and JFK were both existential threats to the military industrial complex and the above the law operations of the CIA.
There was a very strong motive and they had the ability to carry out their immoral deeds.