The purpose of Marlow's trip to the Congo is manifold. On a literal level, if he does not go there, we have no story. On a psychological level, Marlow seeks the heart of darkness, or the deepest recesses of his id (place of secret desires). On a colonial level, Marlow goes there to help the savages. Like his Aunt, Marlow believes in British Colonialism as a means of the white man's redemption. So says essayist and critic Roger Moore:
Thus, Marlow's aunt who arranged his commission with the Company proclaims that the white man's purpose in Africa is to wean the continent's ignorant savages from their ‘‘horrid ways.’’ Marlow himself says that modern efficiency and the ‘‘unselfish idea’’ of conquering the earth, rather than some ‘‘sentimental pretense,’’ is what "redeems’’ the colonial enterprise in which he has been enlisted.
Once there, Marlow's focus shifts on all levels. What once was white (good) is now black (evil). He is awakened to the Empire's and the white man's capacity for evil--even his own. Indeed, he fears soon becoming like Kurtz, who seems to have crossed over to his inner demons entirely.