People will disagree about this, but it is my opinion that the heart of darkness in the book is the darkness that inhabits a nation that colonizes another nation. This is shown on many levels throughout the story. For example, in each encounter, Marlowe finds colonials who try to keep up appearances, but who cannot stop the "rot" within them. That rot is shown throughout the book in the inability of the colonials to truly conquer Africa, and ultimately, of course, in Kurz's madness. It is the conquers who are conquered by their attempt to dominate another land and another people, with ignorance and inhumanity.