In Heart of Darkness, the author shows the European imperialist idea and finds that its outcomes are as dark as the African continent it portrays. The book's two centers are two dark hearts that fascinate Marlow. One is found in Congo,which is the heart of Africa andthe other is held by Kurtz. The two hearts have a bad effect upon Kurtz, who possesses one and wants to possess the other. So Conrad frequently hides them inside of Kurtz. Conrad offers Kurtz as one whose dark nature was provoked by something dark lurking in Africa - two forces that ultimately pull Kurtz from decency and restraint. But "you can't judge Mr. Kurtz as you would an ordinary man" reminds the Russian sailor, and indeed Kurtz is a larger-than-life Nietzschean superhero here. .
The darkness in his heart is so greatly pronounced as to suggest that Kurtz represents the idea of imperialism rather than the common imperialist. Taking Kurtz as an embodiment of the imperialist idea, then one sees that the hearts of imperialism and Africa both contain complementary, destructive darkness. .
The darkness Kurtz holds within himself - mirroring the darkness of the civilizing mission itself - is inadvertently revealed in the brickmaker's comment on Kurtz: "He is an emissary of pity, and science, and progress, and the devil knows what else." Although jocular, this statement hints that there is more truth in the devil's knowing the heart of Kurtz, and that of imperialism, than is visible on the surface. Certainly Kurtz wants to bring some good values to the Congo, and is pursuing the goals of many other civilizers, but so much is unknown about what he brings to accompany progress and enlightenment. One reason that Kurtz is seen as someone who has always had the potential for dark acts is that he has always been seen as a zealot, and the power of zealotry subsumes everything in reach for its purposes - including the oft-mentioned restraint of genuine civility.