It is quite reasonable to believe that Lee Harvey Oswald was the lone gunman who killed Kennedy.

It is also reasonable to believe that Jack Ruby was sent to shut Oswald up, which he did. Ruby had Mob connections and, I believe, was expected to die from cancer anyway.

That still does not answer the questions of who planned the assassination and who knew it was going to happen.

I believe that Lyndon Johnson was fully aware of the plan even if he was not the instigator. Kennedy treated Johnson like he was a dumb hick and despised Kennedy in return.

I think Marcella and the Mob planned and executed the assassination, but Johnson knew it was going to happen and used his influential friends in Dallas to help clear the way.