The History Channel (Or A&E?) had a great program on prior to them airing "JFK" the movie. It basically debunked all of the conspiracy crap and misinformation in the movie and put forth by conspiracy theorists. It was excellent.
Three observations:
(1) Apply Occam's Razor and see how much of this silliness evaporates.
(2) Most conspiracy theorists surmise that JFK was killed by the "military-industrial complex" or the mob or angry Cuban nationalists.
In reality, the Soviets had real motive to kill him and they definitely were in the business of clandestine assassination.
If one really did buy into the conspiracy mindset, the most logical culprit is the Kremlin.
(3) Most people would prefer to believe in a vast conspiracy than face the scary notion that one disturbed individual can change the course of history.