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.