Joseph Conrad's novella Heart of Darkness (1899 and 1902) remains his best-known work, and a classic in the genre. It has been adapted to film twice and is loosely based on a true story in Conrad's life as a British merchant marine Captain.
Despite the far reach of the British Empire into Africa at the time of writing, it remained a mysterious and dangerous place for Europeans. Most believed it to be a place where progress and time had not penetrated, populated by savage tribes and strange animals. The varied atrocities performed there by both natives and colonizing whites were the main influence on Conrad when writing Heart of Darkness.
In the story, the character of Marlow travels into Africa to find a missing man. He is met with horrors in the jungle, including cannibal natives and merciless ivory-hunters; at the time, any white could commit crimes against natives and be condoned by law. Of special note are the well-recorded atrocities of Belgian ivory traders, who were granted carte blanche by King Leopold II to take what they wanted; this removed their need to justify their hunting with "spreading civilization" and so they simply invaded. Marlow recounts his difficulty in finding Kurtz, since the Company men want Kurtz to continue sending ivory, regardless of his other actions.
Another example is the disparity between the native tribes of Africa and the "civilized" Europeans. Marlow compares their interaction by supposing a visit to Ancient Britain by a Roman of the time, and the strangeness that "civilized" man would have felt. Despite an attack on his steamer by natives, Marlow comes to see that the false civilization brought by Kurtz and those like him is in fact more atrocious than the natives, who he had previously considered little more than animals.