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.