How does the work treat the ideas of human morality and human nature?

Joseph Conrad's "Heart of Darkness" explores the moral quandaries encountered by protagonist Charlie Marlow, who has left the safe confines of civilized Europe for a trip through Africa's rough-and-tumble ivory trade. Serving as a steamboat captain on the Congo, Marlow's been tasked by his company with retrieving a fellow employee named Kurtz, along with all the ivory Kurtz has collected so far.
Early on, Marlow identifies European civilization, along with its knowledge and ideas, as a sort of "lightness"—a positive attribute that he uses to define what is most human about us. Conversely, he identifies the lack of such civilization, including the harsh African landscape and its savage citizenry, as a kind of "darkness"—something imperialism and colonialism are meant to rectify. Even the way mother nature is portrayed supports this view as he proceeds further up the Congo into dense, dark vegetation, which appears symbolic of his view toward the locals.
However, Marlow's views begin to change as he glimpses the evilness generated by civilized society's lust for ivory. This is best embodied in the Kurtz character, a product of European civilization who has been corrupted by greed, seduced by power, and ultimately transformed into a thief and a murderer. In essence, Kurtz has become the savage, causing Marlow to reassess his views on "lightness" and "darkness." The imperialistic greed and brutality of Europe slowly come to represent darkness, while the natural wonders and honest reality of Africa come to represent light.
There is a strong question of what it means to be human throughout. While the main character's original notions of humanity were based on the alleged benefits of imperialism itself—knowledge and modernity and technological advancement—relegating the African natives to a sort of sub-human status, that very idea later comes into question when it becomes apparent imperialism itself has turned Kurtz into the type of sub-human savage that was once ascribed only to the local population.
Kurtz has gained success through his immoral behavior, which only begets further immoral behavior. The theme is reminiscent of Lord Acton's late nineteenth-century observation that "power tends to corrupt and absolute power corrupts absolutely." Kurtz's descent into savagery has left him unable to tell the difference between right and wrong. It’s not simply Kurtz’s relentless quest for further material wealth that makes us question his humanity, but his brutal, bloodthirsty methods that show an utter lack of regard for others. His greed ultimately results in pure moral abandonment, showing that all men have both a "light" and a "dark" side and that we are defined not by where we live, but what we do once we’re there.