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 @ RobertkennedyJr, Cesär was a CIA asset.

If this was truly the case & Sirhan was simply an MK Ultra Patsy then you how you know why they would have the 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.

JFK himself told us that he wanted to splinter the CIA into a thousand pieces and scatter it into the wind.
Neither he nor his brother wanted to participate in the Vietnam War which turned out to be a CIA playground for psyop operations and countless crimes against humanity.