As hinted by
@Rallywagon, Kennedy may not have actually been elected democratically, but stole the election through voter fraud in Illinois and Texas. The agents of this fraud were said to be the mob in Chicago, and LBJ's oil connections in Texas. Kennedy subsequently alienated both of these constituencies and they turned on him. Kennedy infuriated the mob by allowing all their business properties, including lucrative casinos, in Havana to be taken over by Castro's revolution. Later, he set his bother, Bobby, the Attorney General, to investigate and prosecute the mob, infuriating them further. The Bay of Pigs invasion fiasco when the CIA mercenary army was slaughtered on the beaches due to denial of air cover by Kennedy, enraged the CIA immensely, and made them another of his enemies. Another sort of betrayal was to his wife, Jackie. Kennedy, despite being the first Catholic president, was a known philanderer and was widely believed to have cheated on his wife many, many times, possibly including Marilyn Monroe, a famous starlet of the day.
Bottom line, Kennedy betrayed his family, betrayed the oilmen and the mobsters who got him elected, and betrayed the CIA who's invasion of Cuba was killed in the crib. He was not loyal to the people who served him. In short, everybody and his brother had a reason to see him dead. The only folks who thought otherwise were believers in Camelot and schoolboys like me who cried when he was shot to death in Dallas.