I think his communistic leanings were real. I have seen enough evidence of that over the years. If I'm not mistaken, two of his closest allies and speechwriters were also explicitly communist.

And large portions of his Letter were plagerized. Including the bits Christians love to quote.

But did the intelligence agencies kill him? Absolutely.

The guys at Haunted Cosmos got into a lot of the details of his murder in their second MKUltra episode, if you are interested: