If there was a conspiracy to assassinate JFK, I have an odd chain of circumstances that might explain *why*, if not who or how.
Today, JFK is regarded as the poster boy for Addison’s disease, among his many other medical problems.
http://en.wikipedia.org/wiki/Addison%27s_disease
While at the time this sickness was not widely known, Addison’s disease can cause, and in JFK’s situation, did cause, very unique and noteworthy changes to the skin of his upper body and face. A physician familiar with Addison’s would spot this immediately from several yards away.
JFK had been taking the only known treatment for Addison’s at the time, cortisone, for many years. It was a very expensive drug until the mid-1950s, when a way was found to synthesize it, so it suddenly became much cheaper, and a “wonder drug”.
However, there were concerns that taking a large amount of cortisone could make a person mentally unstable, even clinically paranoid and dangerous. Even though many doctors believed this back then, it is not true.
Then, in 1956, the top Hollywood actor James Mason, at the peak of his career, appeared in a movie called Bigger Than Life, about a man who had to take cortisone to save his life, but then turned into an insane, homicidal maniac.
His performance was described as “chilling”.
http://www.imdb.com/title/tt0049010/
Finally what set it all off was the Cuban Missile Crisis, and the realization that JFK could on his own, start World War III, resulting in a world conflagration.
Thus the logical chain was complete.
JFK has a disease, the treatment for which requires him to take a large quantity of a drug that could (will?) turn him paranoid and violently insane. And this insanity could cause him to start World War III.
I will add that JFK had been asked to resign because of his medical conditions, and had refused.
Thus, *someone* could conclude that JFK is too dangerous to live.