Marlow journeys not only into the heart of Africa where he sees what isolation and moral corruption have done to Kurtz but he also gradually recognizes what he himself is capable of becoming--a man like Kurtz. The trip up the river becomes a journey into himself, an examination of what he believes and knows is right.
Marlow naively begins his trip from Brussels, seeking only to pilot a steamboat, but upon his arrival in Africa, he is shocked by the condition of the Africans and by the chaos that greets him. Moving further inland, he discovers that the corruption only worsens when he meets characters like the Accountant and the Brickmaker, both of whom represent hollow characters (like ivory) more concerned about appearance than compassion or accomplishment. As Marlow waits to repair his foundered boat, he stresses the value of work. Traveling from the Central Station to reach Kurtz, Marlow recounts his admiration for the cannibals' restraint, which sharply contrasts the pilgrims' greed and eagerness to shoot their guns.
Marlow's journey causes him to examine what he believes, and he realizes that choices are often not easy to make when he must choose to lie to the Intended despite his hatred of lying. He learns that compromise is necessary, but he won't allow himself to step over the precipice into the pit of moral decay where he found Kurtz. Now Marlow knows all men are capable of evil.
Marlow's journey is symbolic of a person's mind and soul. Just as Marlow progresses into the darkness of the African continent, a person's brain and heart are just as dark as the continent itself in that we really don't know what that person is thinking or what that person is capable of. The darkness of a man's soul would be the evil that he's capable of, thus the title. The African continent is uncharted just as a person's mind and soul are uncharted territories.
Some also see Marlow's journey as a metaphor for sin and redemption. As is true in so many books, dark and light are symbolic for good and evil. Marlow's journey into the darkness of Africa is representative of the darkness of human evil and death. Returning from Africa, or the light, then represents life and goodness. Kurtz was unable to be saved from evil, but Marlow is saved in the end. In recognizing all people's penchant for evil, he's able to save himself from it by not giving in to his darker side.
For more examples of symbols in the book, go to the site below.
Could you consider the trip symbolizes one walking through Hell?