How does the text exposes the darkness of human mind?

Despite its use of the N word, which was pretty standard when the book was written in 1899, Heart of Darkness is a devastating indictment of imperalism. In the novel, the Africans are treated as money-making objects; they are habitually ill-treated, punished drastically for almost nothing ... They are whipped, mutilated, hanged.

Moreover, the novel is based on actual conditions in the Congo between about 1880 and 1908 ... It is generally agreed that at least 3 million (!) Africans were killed by genocidal colonists.

The novel ends in tragedy, as suggested by the title Heart of Darkness, referring primarily to man's inhumanity to man as witnessed in Africa. When Marlow discovers the truth about Kurtz's greed, ambition, and inhumanity, he realizes that there is no vestige of pure motive in the European imperialism in Africa except greed and evil triumph. And yet Marlow cannot fully come to terms with this knowledge because of his strong idealism and belief that there must be goodness in mankind. At the end of the novel it is obvious that he cannot fully accept the truth, and he lies to Kurtz's Intended and lets her believe that Kurtz died with a noble purpose, helping the Africans, and uttering her name as his last words.