Everyone with a brain knows: The Mob had JFK killed by Oswald as payback for JFK's crawfishing on the deal Joe Kennedy Sr. made to put JFK in office.
Joe Kennedy Sr. wanted to be PotUS, but had too many Mob connections in his closet to run himself. So, instead, he chose to be The Mandarin -- let his kids run, then rule through them. First Joe Jr.; then JFK; then RFK; then perhaps Edward, or some of the Kennedy clan grandchildren (if Joe Sr. lived that long).
The plan took a bullet when Joe Jr.'s assignment to a rear-echelon project proved rather more dangerous than anticipated -- who knew trying to fly a B-17 filled with explosives might be a hazard to one's health? (Look up "Project _Aphrodite_".)
So the Heir Apparent is gone. Worse, the second in line managed to get ambushed by a Japanese destroyer, losing half his PT-boat crew. It took some serious polishing to get that turd to shine; but finally, shine it did, in time for the '60 PotUS election. That he was running against The Least Charismatic Man In Political History in '60 helped a bit, but not quite enough for Joe Sr.'s liking.
So Joe Sr. calls in some of his markers -- the dead rise to vote en-bloc for the Democrats in Chicago, and it's just enough to put JFK over.
Problem: JFK and RFK were never told of Joe Sr.'s machinations -- nor of the bargain Joe Sr. had to cut: "We put your boy in the White House; you tell him to back off of us." Before Joe Sr. can act, JFK installs RFK as Attorney General, and turns him loose on the Mob.
Worse: JFK's complete botching of the Cuba operation guarantees the Mob will never see its Havana-casino income again. Rule of Thumb: NEVER cost a Mobster money....
The Mob finds a Useful Idiot in Oswald -- someone with Cuban connections, however tenuous; and who can decapitate a rabbit at a dead run from 300 yards. Making the SBD shot is child's-play for him.
"First Rule Of Assassination: Kill the assassins." The Mob arranges with a terminally-ill club owner: "You kill Oswald, and keep your trap shut afterward, you go to jail; but you get free medical care for the rest of your life, and we'll make your stay as comfortable as possible." Oswald dies shortly thereafter. Ruby spends some time in the pen, and dies quietly.
(And when RFK shows he didn't get the message -- well, you figure it out.)
[sigh] Oh, for the days when the only mobsters f***ing with PotUS were American mobsters....