Fidel Castro Angered that Kennedy backed the Bay of Pigs Invasion so he wanted revenge
The Mafia: The Mafia was upset with him because after their backing him in the 1960 election, he turned on organized crime and assigned his brother, Robert, the Attorney General, to establish a task force to prosecute them. Remember, Jack Ruby was alleged to be involved in organized crime!!
The CIA The CIA, because they feared what he was capable of doing on the international stage, and bungling things for America as he did in the Bay of Pigs Invasion