JFK was killed because he made promises to Giancana and didn't keep his word after Sam helped him get elected.
It is also speculated that Murray Humphreys played a role in swinging John F. Kennedy's 1960 election, leveraging the mob's influence in key states.
This falls under "Durable Myths of the Mafia." Let's take a look at reality:
Myth: Joseph P. Kennedy, JFK’s father, had ties with organized crime going back to Prohibition, when he was a major rumrunner, and kept up with them after Repeal. In 1960, he contacted Sam Giancana, to solicit money and support from the Outfit and its ally, the Teamsters Union, to help his son win Illinois in the general election. In return, Kennedy promised that his son’s administration would keep hands off organized crime. Giancana agreed to help. But JFK appointed his brother, Robert, as Attorney General. Bobby Kennedy declared war on the Mob and on the Teamsters. So, the conventional wisdom states, JFK had to be killed as revenge, and to keep the government off of the Mob’s back.
Another wrinkle: Giancana and JFK shared a mistress: Judith Campbell Exner, a model and would-be actress, introduced to both by their mutual friend, Frank Sinatra. Later, Exner, terminally ill with cancer, alleged that JFK had used her as a go-between to deliver cash to Giancana to fund assassination attempts on Castro, and to have Giancana arrange for her to have an abortion.
Reality: Joe Kennedy never was a rumrunner during Prohibition. That myth got started because, in 1932 when Prohibition was about to be repealed, Joe Kennedy formed a partnership, Somerset Importers, and in '33, after Repeal, started bringing in, legally, several brands of scotch and gin from the UK. His partner in Somerset Imports: James Roosevelt, FDR's son.
Giancana had no motivation to help JFK carry Illinois. In 1957, he was subpoenaed to appear before the Senate Permanent Investigation Subcommittee, which was looking into OC infiltration of labor unions. JFK was a Dem on the subcommittee, his brother Bobby was chief counsel. Giancana took the Fifth Amendment each time he was asked a question, and gave out a little laugh. "Why are you giggling, Mr. Giancana, Bobby asked him in front of the TV cameras, "I thought only little girls giggled." You don't say that to a Mob boss in front of millions of viewers and expect him to support your brother for President. That hearing also marked the opening of the long war between the Kennedys and Teamster's President Jimmy Hoffa, with JFK shouting at him: "Why don't you stop hiding behind the Fifth Amendment and answer questions truthfully." Hoffa and Bobby almost came to blows at the hearing. The Teamster supported Richard Nixon in 1960.
JFK didn't need the Mob or the Teamsters to carry Illinois because it was delivered to him by his friend and ally, Richard Daley, Chicago's all-powerful Mayor, by the time-honored tactic of having the dead vote early and often. Nixon was co8mfortably ahead in Illinois until 11 p.m. when a huge chunk of JFK votes came in from Cook County. JFK carried Illinois by 8k votes.
As for Exner: JFK didn't know he was sharing her with Giancana until FBI director J. Edgar Hoover informed Bobby that Exner had called the White House switchboard more than 30 times in two weeks--and that one of the calls was traced to a phone in Giancana's home. JFK immediately dropped Exner, and Sinatra, and had no contact with either after that.
