How is Heart of Darkness an angry document of brutal exploitation-a record of things seen and done?
The novella certainly is a comment on the evils of materialism and the destruction of humanity and good that can occur when human beings quest for such materials things at any costs--"the horror" that exists within each of us if our own greed is not kept in check. However, I'm not sure we can say that it is a completely accurate record of the exploitation of Africa.
Marlow as a character and as a narrator is part of the Imperialism that seeks to exploit the situation for his own personal and material gain. Although he has certainly not sunk to the same depths as Kurtz, he actions and attitudes only help to contribute and expand the problem. Although he personally may have mixed feelings about the exploitation that he witnesses, he never takes action to correct or avoid such behavior and never speaks out against it.
For a fascinating perspective on this topic, I'd encourage you to take a look at some of the criticism
Joseph Conrad based his novel on first-hand experience. That is what makes it so powerful. Although it is fiction, the images and situations presented in the novel are meant to call attention to the evils of colonialism, the evils of one culture imposing its will on another, the even worse evils of treating people as if they were less than human. Indeed, in this novel, some of the Africans are treated worse than animals. As Marlow travels into the heart of Africa on the Congo River, he sees multiple atrocities and when he finally locates Kurtz, it is even worse. The horror!
If you read the themes section at the link below here on eNotes, you will find some specific information about the themes in this novel to help you formulate a more complete answer. This novel is very disturbing because it shows the evil that man is capable of and hence the title, "heart of darkness."