Why was the United States involved in Patrice Lumumba's assassination?

Even in the late fifties, "the communists are coming..."paranoia had grips on the US, fanned by partisian politics of the far Right.
It isn't clear to me that the CIA was involved directly in the acutal assination, although they had prepared to do it.

http://www.usnews.com/usnews/doubleissue/mysteries/patrice.htm

So Patrice was spreading communism and the you think that the CIA killed him so that leads up to the suspicion of the United States?

Patrice was spreading communism. It is not clear the CIA killed him. Read the link I posted.