He helped Castro in Cuba, didn't he? And then he moved on to...Bolivia? Probably the association with Castro/Cuba itself made the USA view him a Commie; whether he was or wasn't, I don't really know. They were probably afraid that he'd spark off more Communist revolutions in South America, which is the USA's "backyard", so to speak. Monroe Doctrine or something, I think. (Not sure!) And they would have pulled out the same old tricks, the Domino Theory, after Cuba fell the rest of South America would fall to the great evil that is Communism. Ergo, the CIA conveniently dispose of him. It's plausible; they usually work by proxy, anyways.

I think.