Assuming that Oswald was a lone assassin, the best explanation is that Oswald forgot to bring his revolver that morning -- but that does not explain his apparent lack of a plan for escape after Kennedy was shot. Maybe he intended to surrender at the scene, but in the moment, decided to flee instead.
Or, whatever Oswald was involved in, the shooting of Kennedy was not what he signed up for. Perhaps Oswald was on his way to the Texas Theater as a designated meeting place but wanted his revolver first. The shooting of Tippit on the way was most likely an impulsive act driven by a sense of panic.
Chapman was diagnosed by defense experts as psychotic and claimed to draw inspiration from the book Catcher in the Rye. In contrast, Oswald was not mentally ill even if his belief in Communism made him what is commonly called a nut, meaning someone who overvalues his ideas and goes too far in acting on them. And comparing Oswald to Chapman as you do gets back to my point that if things went as planned, Oswald could have surrendered at the scene as Chapman did.
As for conspiracies, the world in awash in them because humans are social creatures who commonly seek out and find collaborators for large tasks. Most conspiracies though do not go beyond talk and minor gestures, many of the balance miscarry in application or get exposed, and of course, not everything that may seem to be a conspiracy really is or the true dimensions of the conspiracy are not recognized.
For now, the evidence for a conspiracy in the Kennedy assassination is suggestive but elusive as to definitive proof for or against. In great part this is because the FBI investigation and the Warren Commission avoided diligently looking for and pursuing evidence of a conspiracy.
The later House Special Committee investigation was handicapped not just by the passage of time but also by political infighting, CIA obstruction, and the deaths of key witnesses. For example, George de Mohrenschildt, an Oswald friend with suspected CIA ties, killed himself the day he was to talk with investigators, and the Mafia figures Sam Giancana and Johnny Roselli were murdered shortly before investigators were to speak with them.
Notably, the CIA falsely denied to the House Committee that David Atlee Phillips used the cover name "Maurice Bishop" and also lied about its support for the anti-Castro exile group DRE. A few years ago, a Washington Post reporter pursuing FOIA litigation was able to depose a CIA officer who described Oswald as being of intense interest before the assassination at the highest levels of the the CIA's counterintelligence unit.
At the very least, even for those who adamantly believe Oswald was a lone gunman, the historical record of who Oswald was is deficient in that we do not know how Oswald was viewed by the CIA before the assassination and if the ongoing spy game in some manner spurred him to action or was responsible for a lapse in Presidential security.
The most innocent explanation for the CIA's obstruction -- a conspiracy of sorts -- is that it wanted to avoid the institutional damage that it would suffer if the details were revealed about how closely they were monitoring Oswald. Plausibly, the CIA may have let Oswald elude adequate attention as a risk to Kennedy's safety despite knowing in advance through de Mohrenschildt or Phillips or others that Oswald had bought a rifle, had shot at Edwin Walker, and was prone to and capable of another violent act.
If the CIA did not warn the FBI and Secret Service when a Presidential motorcade planned to go past the building where Oswald worked, they would be suspected of complicity in the assassination. The CIA's defense would be to argue, no, really, we are honest patriots but we simply bungled and let Kennedy get killed even though we could have stopped it. Would you want to try making such a defense on behalf of your agency, or would you stonewall investigators?
Moreover, as the deaths of Giancana and Roselli suggest, the Mafia may also have done some housecleaning rather than risk letting secrets out about the Kennedy assassination. Perhaps Oswald and Ruby -- both of whom had mob ties -- were part of a Mafia conspiracy to kill Kennedy. Oswald, the Commie nut case, would have been especially useful to divert attention from the Mafia. Or maybe Giancana and Roselli were killed because of their role in the CIA's plots against Castro -- or for entirely unrelated reasons.
With the historical record incomplete, there is reason to suspect the CIA or a rouge element affiliated with it had a role in killing Kennedy. Nor can LBJ be excluded as a conspirator, in conjunction with the Mafia and CIA affiliated figures and financed by Texas oil interests.
Notably, CIA Director Richard Helms called in a close friend and ally within the agency and appointed him to internally investigate the Agency's handling of Oswald. Yet the man was removed as soon as he started probing in sensitive areas in earnest. His replacement generated a report that is seen as a whitewash. Helms seems to learned what the CIA's role as to Oswald was and calculated that the agency could not bear being made public.
Those who privately suspected or knew that there was a conspiracy in the Kennedy assassination include LBJ, Richard Nixon, Bobby Kennedy and Jackie Kennedy, Peter Lawford, E. Howard Hunt, John Mitchell, Henry Cabot Lodge, most members of the Warren Commission, and both Soviet and French intelligence.
I think that eventually, there will be a definitive resolution of the Kennedy assassination, probably long after we are gone and historians get access to documents that are currently classified or otherwise withheld from public view. Until then, as interesting the subject as it is -- we dare not overvalue our ideas about it!
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