Marlow means that the Kurtz' search for ivory has gotten the better of his judgement. He had become so greedy for the product that he had set up his own little kingdom inside the Congo. He had the natives working for him alone, not for the company that Kurtz supposedly worked for. Because of that, Kurtz had lost his own humanity and began to be a "big fish in a small pond", so to speak. He separated himself from the rest of the Europeans and began to identify more and more with in savage ways. He became totally immersed in his own world, and lied to himself that the other world did not really exist. As a result, he also lost his sanity and eventually his life because he did not remember what his original intentions were.
Marlow, whose hatred for lies does not allow him to become enveloped in the Congo, eventually returns to his own culture to tell us Kurtz' sad tale.