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Hear me out.
Marylin Monroe was about to go public about her affairs with the Kennedys and the state secrets they told her. (Castro, Cuba, The Abortion Bobby forced her to get, etc) She was killed by the mob on orders from Bobby Kennedy (who watched the hit) in exchange for him getting off there back. JFK was killed by the CIA because he wanted to end the Federal Reserve and expose the Wealthy elite that secretly run things. Bobby was killed by the CIA because he wanted to finish what his brother started. MLK was killed by the FBI/CIA because they were scared he would unify the country against the powers that be. Regan was shot to keep him from following the Kennedys in exposing the elite.

Joe Demagrio and Frank Sinatra were just caught in the middle and trying to keep Marylin out of trouble. They of course weren't able to remarry/marry her before she was wacked leading to both men living with the guilt of her death for decades.
Or we could all be wrong and the Studio wacked her. We'll never know 100% unless someone makes a Deathbed confession, the red and/or the black diary turns back up (unlikely since they would have been destroyed by whoever stole it), or some damaging documents get released.