I believe the conspiracy theories are there because the Left wants us to believe that Right Wing forces killed Kennedy...Kennedy was a hardcore anti-communist right-winger when it came to foreign policy.
You need to do your homework and where and how and why Oswald learned Russian.
Answer: he was recruited and trained by the CIA to infiltrate Russia while he was a Marine in Japan. He was not the only one, btw.
His handler was counter-intelligence chief Angleton.
Newman’s “Oswald and the CIA” will walk you through this CIA document by CIA document—boring but worth your time.