I think all the mentioned people had ties or friends in organized crime and/or had 'business' with the Kennedys. Kilgallen I think just did an interview with or had information on Jack Ruby who was involved in the JFK assassination/killed OswaldI read a long time ago that her death was not really a suicide. One theory was that she and Dorothy Kilgallen were planning to reveal some information about UFOs to the press that certain powers that be didn't want released. Their meeting was already scheduled when Marilyn was found dead in August 1962. Ms. Kilgallen was found dead in her apartment November 1965 of undetermined cause, but barbiturates and alcohol combo were suspected. Now they're saying that George Reeve's (2nd actor to play Superman) death wasn't a suicide either. I think it's a shame when false information about suicide being the cause of death gets perpetrated.
Some say Monroe was hoping for a more open or serious relationship with one of the Kennedy brothers. A public relationship which many already knew about would've ruined 'Camelot' and re-election.