As a former LAPD Homicide detective, I am always fascinated by the seeming public allure of conspiracy theories. As a detective, you try to uncover the facts and find proof of who committed the crime. The emphasis here is on the word 'proof.' Whether it is through interviews, forensic evidence, witness identification etc., detectives are constantly trying to find who actually did the crime. The beauty of conspiracy theory adherents is that they don't have to prove anything, just come up with multiple theories to discount whatever evidence or proof has been gathered. I dare say that there are more conspiracy theories about the Kennedy assassination than probably any crime that was ever committed.
In almost all of these theories, there is this malevolent unknown mastermind who brilliantly set up the most complex web of deception, including numerous small time bit players, that were not only able assassinate an American president, but then masterfully kill the person who the authorities blamed for the crime, Lee Harvey Oswald. It's as if Professor Moriarity, Sherlock Holmes arch nemesis, pulled off the greatest crime of the century without anyone knowing of his existence.
I find it amazing that with all the bit players and innumerable pieces of the puzzle that would have had to be in place to accomplish this, not one person has ever revealed him or herself to be part of the plot or shown any evidence to prove it. Think of the notoriety, not to mention the financial reward that a person would have if they did this, yet no one has ever come forward. Conspiracy theorists will usually be quick to mention that the key players were all eliminated. They would want us to believe that the landscape is full of unsolved murders and mysterious disappearances that law enforcement agencies were not only unable to solve but in actuality were linked to this vast, complex conspiracy.
I think in many cases the forgotten key to this assassination was the killing of Dallas police officer J.D. Tippit. Immediately after the president was shot, it was believed that the assassin had fired the shots from the Texas Book Depository. The rifle used to kill Kennedy was recovered from the 6th floor of the building. A description of the suspect had been broadcast to police units throughout the vicinity. Officer Tippit stopped a man fitting the assassin's description a few blocks from the residence that Lee Harvey Oswald had rented under an assumed name. After the officer exited his vehicle and approached him, the suspect shot the officer a number of times and then fled the scene on foot. Multiple independent witnesses later identified Oswald as the man who shot the officer and fled the scene. Oswald was later apprehended in a movie theater not far from the Tippit shooting. During the struggle to arrest him, Oswald attempted to fire a gun but it misfired. After he was apprehended, it was discovered that Oswald worked in the Texas Book Depository.
In closing, I think a lot of people who doubt Oswald killed Kennedy find it difficult that such a marginally small and troubled man, and not a vast, complex web of deceit could have so radically changed the course of American history in those few seconds in Dallas, Texas in 1963.