Joseph Conrad's Heart of Darkness represents an ironic reinterpretation of who holds the qualities of light and dark. The speaker, Marlowe, has all the standard belief of an Englishman of his time in London as the greatest city on Earth; in real work being done in giving the advantages of civilization to the native peoples of Africa; and in the superiority of European intelligence and virtue over the primitive ways of those whao are native to the African continent. In an ironic twist of fate, Marlowe comes to compare the integrity and dignified behavior of the cannibals traveling on his ship to the behavior of the Europeans on his ship for the same journey. He also comes to compare his work and self-seeking to the work and indolence of the Europeans in the settlement where he is repairing his broken ship. Finally, he compares the reputation of Kurtz to the reality of Kurtz. These comparisons further generalize to London and then to European civilization.
The end result is that Marlowe perceives a true inner integrity in the cannibals that is shockingly missing in the Europeans; this assigns "light" to the cannibals on the ship and "dark" to their European co-passengers. The result of Marlowe's comparison of his work, which opens his soul to him and constructs his sense of self and worth, to the indolent greedy attitude of the Europeans running the settlement is that Marlowe, and what he stands for, which is personal integrity and devotion to a factually beneficial cause, is "light" while the ivory collectors and labor exploiters are "dark." When he discovers the truth of Kurtz's condition and stands by his side as Kurtz dies, the vapor of Kurtz's image as a bringer of "light" evaporates and Kurtz is shown to be "dark" after all. Finally, London, and by extension European civilization, which is touted as being "light" is revealed to be in fact "dark," as is proved by the results of England's and other European missions and actions.