Why does Marlow become disillusioned with kurtz being on the boat?

There has been an aura of mystery around Kurtz since his introduction as a character. Marlow is usually too distant to be affected by this sort of thing, but he is drawn into it, as well. Kurtz was fascinating to him theoretically, but on the boat, Kurtz is all too human. Kurtz also reveals himself to be an opportunist and perhaps something of a con artist.