The JFK assassination is the only conspiracy theory that I actually believe has a lot of merit. After reading a few books on the (underground) politics of the era here is what I believe to a good degree of certainty happened:

* The hit was ordered by the Chicago mafia boss at the time, Sam Giancana

* CIA wasn't directly involved but caught a whiff of the plot and let it happen anyway because they were very unhappy with JFK

here is why the mafia was unhappy with JFK:

* JFKs father made his fortune as a bootlegger during the prohibition era and was very friendly with the mobsters

* the OG mobsters wanted JFK as a president because they were going to have some influence on him through his father.

* the mafia and specifically Sam Giancana helped JFK getting elected in Chicago which was kind of a flip state

after getting elected JFK was not having any of it and completely cut off the mob and actually went against them in a couple of ways

Firstly his brother Robert Kennedy was made attorney general and went after the mob hard. He made his goal to put Jimmy Hoffa behind bars. Hoffa was mobbed up and it was the main financier of the mobs building casinos by providing loans to them through the Teamsters Pension Fund.

The mafia had built many casinos in Cuba. After the revolution, Fidel Castro nationalised the casinos taking away the big cash flows from the mob. The mafia wanted JFK to invade Cuba and take down Castro so they could take control of the casinos back.

here is why CIA was against JFK:

* The bay of pigs invasion was orchestrated by the CIA by training Cuban anti-Castro dissidents that fled the country after the revolution and sending them to fight during the invasion.

* During the invasion JFK promised that he would provide air support to the troops but at the last moment he flinched leaving the ground troops helpless which doomed the operation

What I believe happened:

* Oswald was set up as a patsy for the assassination. His handler from the mob was a guy named Jack Ruby.

* Jack Ruby was a known mobster and the right hand man of Sam Giancana

* When Oswald was captured and during his transport after the initial interrogation Jack Ruby killed Oswald live on television (you can find it on youtube) so he would not blow the whistle

From my understanding the main falling out in the relationship of JFK, the CIA and the mob was when he intentionally botched the invasion to avoid an all out war. This is what lead to his assassination