Various conspiracy theorists put together some circumstantial evidence (JFK and Giancana shared the same mistress, Giancana supposedly helped JFK carry Illinois, Mafia helped CIA in plots to kill Castro, Mob became enraged when Bobby K turned on the Mob and deported Carlos Marcello) to "prove" that the Mob killed JFK. I don't think so for many reasons. Most important: Organized crime had no reason to believe it would get favorable treatment from the Kennedy administration. In 1959, during Senate Labor Committee hearings on organized crime influence over unions, JFK, the committee’s chairman, and RFK, the chief counsel, grilled Mob figures relentlessly. RFK purposely humiliated Giancana by likening him to “a little girl.” He also made a lifelong enemy of Jimmy Hoffa. Not surprisingly, the Teamsters Union, and their Mafia backers, supported Richard Nixon in 1960.
And if they did want to kill a Kennedy, why would they choose JFK, when RFK was their nemesis—-and leave RFK, who was notoriously ruthless, to wreak vengeance from his position as Attorney General? Why would they use a certifiable nut-case like Oswald as an assassin, when they had plenty of better killers in their own ranks? And why would they have Oswald killed by another certifiable nut-case like Ruby—-and leave him behind to tell about it?