Both Marlow and Kurtz are symbols of the mechanism by which colonization took place and was sustained. A trope of much colonialist literature and of representations of the European experience in Asia and Africa is an attraction-repulsion the white man feels toward "the natives" and their cultures. In George Orwell's Burmese Days, for example, there are constant allusions to Flory's sense of how alien, how disturbing, the Burmese culture is to him personally. Yet he has come to see Burma as his "home" and does not want to go back to England. In Heart of Darkness the scenario is a more spectacular (and horrifying) manifestation of the same thing. Kurtz has gone mad, merging himself with the "native's" culture so completely that he has set himself up as a god over the people.
Marlow, fortunately, retains his sanity but arguably the same mystical and pathological connection with the colonized country is affecting him as well. Something drives him on into the interior to uncover the terrifying mystery of Kurtz that is lurking there. Marlow knows from the start that the whole project of European imperialism is wrong. The dysfunctional situation he sees upon arrival in Africa is described in detail. The constant obstacles and dangers of travel to the interior to find Kurtz would, one would think, deter any sensible man from continuing this journey. Marlow is driven on by an inner demon similar to, but a mere shadow of, the one that has animated Kurtz and plunged him into psychosis.
If the two are both manifesting a disturbed mental state of whatever degree, Conrad's ultimate message is that European colonization is, in some way, a form of psychosis. The mindset behind it is a detachment from reality and the delusion that one people have a right to "take over" or control another. Kurtz's self- deification is an exaggerated, distorted and bizarre version of the belief-system underlying imperialism.
It is interesting that although this is the "moral" of the story, it is never stated openly and unequivocally by Marlow. The whole intricate style of Conrad's prose is an analogue to the mysteries and complications of the meaning of this tale. It is as if the characters are existing in some fog-like opium dream in which reality and illusion are merged. For all the intelligence with which Marlow views the situation, his ultimate reaction is a puzzled horror which shows both his distinctness from Kurtz's actions, and paradoxically his resemblance somehow to the inner soul of Kurtz.