That Oswald shot Kennedy is not in doubt. The question of were there other actors in the killing is one that sadly can never be answered due to the wholesale destruction, and loss of physical evidence.
If others were involved (CIA, mafia, Cuba, Russia) my only question is....Why did they suck so bad?
1. If you are killing the President of the US, why would you entrust something so serious to a nutjob like Oswald? Oswald was unstable and untrustworthy. He would be your choice for an assassin?
2. If you wanted to kill a President, you want it done right. Why would you have Oswald do the job with a shitty, mail order, $19.95 bolt action rifle? Wouldn't you buy him a top of the line sniper rifle?
3. If there were multiple shooters, why did they only manage to get off three shots? Wouldn't they have set up a kill zone and peppered the car with shots?