To answer this, we need to come to an understanding of the Western attitude of Conrad's period regarding the colonial empires that had been established.
One often hears Africa described in the European mindset as the "dark continent." Despite the slave trade and exploitation of Africa that had been going on for several hundred years, Europeans still, by the end of the nineteenth century, knew little, if anything, about the interior of Africa. In Heart of Darkness, the disappearance of Kurtz deep within the unnamed country of Marlow's visit is emblematic of this mystery the Western mind had nurtured concerning not just Africa but any of the lands peopled by non-whites. But "darkness" also refers to the darkness that exists within Kurtz's own mind and soul. What is it, Marlow keeps wondering, that this man (about whom there is a mystical and dangerous aura) is actually doing in the interior? What has motivated him, and what has created the lurid notoriety with which Kurtz has been invested? The revelation that Kurtz has gone mad, that he has set himself up as a kind of king or even a god over "the natives," is like an apocalyptic vision of destruction.
Kurtz's psychosis, the "darkness" inside him, is a metaphor of the darkness at the heart of the European effort to conquer and to control other peoples. Though he does not say so explicitly, Conrad, through the persona of Marlow, has grasped the futility of the white man's dominion in Africa, if we may paraphrase Orwell's observation in "Shooting an Elephant" about the British control of Asia. The irony, however, is that Conrad, like Orwell, seems more obsessed with the effect this dysfunctional arrangement has on the Europeans than its effects on the non-white peoples. The "darkness" of his tale is therefore a multi-layered phenomenon in which even those who are critics of imperialism may themselves have been blinded as to the actual nature or significance of the colonial system's criminal dysfunctionality.