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Lots of people have given their answers to this question. Most of these ideas are whacky. Most of them are disinformation and can be ignored. Why should you consider my answer and not the others? You should consider it because it is the most probable, by a wide margin.
Cicero asked the question: Cui bono? Who benefits? We must ask who benefited most from John F. Kennedy's untimely and tragic demise in Dallas that 22 November 1963.
Who benefited most? It was not Fidel Castro, Nikita Khrushchev, Sam Giancana, Jimmy Hoffa, Lee Harvey Oswald, or the man in the moon. It was someone else.
There is only one person who best fits the bill. He was very jealous of JKF. And he didn't much like the brothers Kennedy either, although he did like the First Lady quite a lot. He was an infamous womanizer, just like the Kennedys. He had a checkered past that some say involved murder to advance his political career. He was a master manipulator. He was the best friend that Israel ever had — even better than Harry Truman. Rumours are that he gave the Israelis nuclear weapons after the 6-Day War which had all been arranged for the Israelis by U.S. intelligence. He gave up to Israel its attack on the U.S.S. Liberty, a spy ship stationed off the Egyptian coast at that time. He controlled the investigation of the Kennedy assassination. He would have known James Angleton, the head of counter-intelligence before and after the assassination. He got all the perquisites of power after the assassination. He became known by a name which rhymes with JFK, the man whose life he stole. He was so bad that he was eventually totally disowned by the Democratic Party (which then worked to blame everything that was wrong with the United States on Richard Nixon. Nixon worked hard to help them.)
James Jesus Angleton, who was the head of counter-espionage at the CIA, arranged everything for the prime suspect. Angleton created a mystery that could never be unravelled. Angleton knew the Israelis because Angleton had created Mossad for them a few years before. Security was relaxed in Dallas by someone with authority to do it. The route was changed at the last minute. Who were the shooters on the grassy knoll? There were some. They weren't the Mafia, or the Russians, or the Cubans. Who assassinates by sniper? Who was Jack Ruby, AKA Jacob Rubenstein?
The CIA was up to its neck in the assassination. The head atop that neck was Angleton. All the other hypotheses are disinformation. They exist to conceal what happened. The only man who benefited most was Lyndon Baines Johnson. He was the master of master manipulators. He was the man who could arrange everything. No one would dare accuse him. It would have been unthinkable, unimaginable. It was the perfect crime.