My favorite theory:

Israeli conspiracy
The Israeli government was displeased with Kennedy for his pressure against their pursuit of a top-secret nuclear program[90][91] and/or, the Israelis were angry over Kennedy's sympathies with Arabs, and his use of men formerly under the employment of the Nazis in their rocket program, such as Wernher von Braun. Gangster Meyer Lansky and Lyndon B. Johnson often play pivotal roles in this conspiracy theory as organizing and preparing the hit, thus bleeding into and possibly catalyzing many of the other conspiracies as well.[92]