Either that or his role in the Bay of Pigs invasion disaster in Cuba.
I've heard so many theories.
* The Mafia did it, because RFK was going after them.
* The South Vietnamese did it, because JFK had a hand in the assassination of Ngo Dinh Diem.
* The Mossad did it, because JFK tried to prevent Israel from getting The Bomb.
* The Soviets did it, because JFK was a Cold Warrior (e.g., the Berlin Wall speech, the Cuban Missile Crisis).
* JBJ did it, because he wanted to be president. (No coincidence JFK was killed in Texas.)
But the most convincing argument I ever read was in Liberty magazine for The Lone Gunman Theory. The article presented a pretty impressive case.