Hypocrisy of Imperialism
Marlow goes to the inner station and sees the horrible treatment of the Natives- harsh picture of Colonial Enterprise. Treatment of Natives as a way to create "civilization" in them was really destroying them. Taking ivory by force. The Natives become a piece of scenery, not as humans- Marlow sees the Helmsman as a piece of machinery and the Black Mistress as a statue.
Maddness caused by Imperialism
Africa is responsible for mental disintegration as well as physical illness. Kurtz has gone mad due to a power trip. Madness, in Heart of Darkness, is the result of being removed from one's social context and allowed to be the sole arbiter of one's own actions. Madness is thus linked not only to absolute power and a kind of moral genius but to man's fundamental fallibility: Kurtz has no authority to whom he answers but himself, and this is more than any one man can bear. Think Kurtz and Fresleven- good powerful people who became mad in the jungle.
Absurdity of evil
This novella is, above all, an exploration of hypocrisy, ambiguity, and moral confusion. Shows how you have to choose the lesser of two evils. Marlow watches the pilgrims kill the Natives with no apparent reason. Kurtz madness and power trip are all examples of how the jungle is a corrupting force and changes the mindset of the people in it.