Marlow's story in Heart of Darkness takes place in the Belgian Congo, the most notorious European colony in Africa for its greed and brutalization of the native people. In its depiction of the monstrous wastefulness and casual cruelty of the colonial agents toward the African natives, Heart of Darkness reveals the utter hypocrisy of the entire colonial effort. In Europe, colonization of Africa was justified on the grounds that not only would it bring wealth to Europe, it would also civilize and educate the "savage" African natives. Heart of Darkness shows that in practice the European colonizers used the high ideals of colonization as a cover to allow them to viciously rip whatever wealth they could from Africa.
Unlike most novels that focus on the evils of colonialism, Heart of Darkness pays more attention to the damage that colonization does to the souls of white colonizers than it does to the physical death and devastation unleashed on the black natives. Though this focus on the white colonizers makes the novella somewhat unbalanced, it does allow Heart of Darkness to extend its criticism of colonialism all the way back to its corrupt source, the "civilization" of Europe.
Heart of Darkness portrays a European civilization that is hopelessly and blindly corrupt. The novella depicts European society as hollow at the core: Marlow describes the white men he meets in Africa, from the General Manager to Kurtz, as empty, and refers to the unnamed European city as the "sepulchral city" (a sepulcher is a hollow tomb). Throughout the novella, Marlow argues that what Europeans call "civilization" is superficial, a mask created by fear of the law and public shame that hides a dark heart, just as a beautiful white sepulcher hides the decaying dead inside.
Marlow, and Heart of Darkness, argue that in the African jungle—"utter solitude without a policeman"—the civilized man is plunged into a world without superficial restrictions, and the mad desire for power comes to dominate him. Inner strength could allow a man to push off the temptation to dominate, but civilization actually saps this inner strength by making men think it's unnecessary. The civilized man believes he's civilized through and through. So when a man like Kurtz suddenly finds himself in the solitude of the jungle and hears the whisperings of his dark impulses, he is unable to combat them and becomes a monster.
Heart of Darkness plays with the genre of quest literature. In a quest, a hero passes through a series of difficult tests to find an object or person of importance, and in the process comes to a realization about the true nature of the world or human soul. Marlow seems to be on just such a quest, making his way past absurd and horrendous "stations" on his way up the Congo to find Kurtz, the shining beacon of European civilization and morality in the midst of the dark jungle and the "flabby rapacious folly" of the other Belgian Company agents.
But Marlow's quest is a failure: Kurtz turns out to be the biggest monster of all. And with that failure Marlow learns that at the heart of everything there lies only darkness. In other words, you can't know other people, and you can't even really know yourself. There is no fundamental truth.
In a world where truth is unknowable and men's hearts are filled with either greed or a primitive darkness that threatens to overwhelm them, Marlow seems to find comfort only in work. Marlow notes that he escaped the jungle's influence not because he had principles or high ideals, but because he had a job to do that kept him busy.
Work is perhaps the only thing in Heart of Darkness that Marlow views in an entirely positive light. In fact, more than once Marlow will refer to work or items that are associated with work (like rivets) as "real," while the rest of the jungle and the men in it are "unreal." Work is like a religion to him, a source of support to which he can cling in order to keep his humanity. This explains why he is so horrified when he sees laziness, poor work, or machines left out to rust. When other men cease to do honest work, Marlow knows they have sunk either into the heart of darkness or the hollow greed of civilization.
Students and critics alike often argue about whether Heart of Darkness is a racist book. Some argue that the book depicts Europeans as superior to Africans, while others believe the novel attacks colonialism and therefore is not racist. There is the evidence in the book that supports both sides of the argument, which is another way of saying that the book's actual stance on the relationship between blacks and whites is not itself black and white.
Heart of Darkness attacks colonialism as a deeply flawed enterprise run by corrupt and hollow white men who perpetrate mass destruction on the native population of Africa, and the novel seems to equate darkness with truth and whiteness with hollow trickery and lies. So Heart of Darkness argues that the Africans are less corrupt and in that sense superior to white people, but it's argument for the superiority of Africans is based on a foundation of racism. Marlow, and Heart of Darkness, take the rather patronizing view that the black natives are primitive and therefore innocent while the white colonizers are sophisticated and therefore corrupt. This take on colonization is certainly not "politically correct," and can be legitimately called racist because it treats the natives like objects rather than as thinking people.