Marlow's journey is usually seen as symbolic of a journey into the mind and soul of a person. Light and dark are used to represent good and evil. As Marlow progresses into the African continent, he's going into darkness. This darkness of Africa then can be representative of the darkness of a person's soul. Marlow learns on his journey that all men are capable of evil, and he sees this in Kurtz and others. We see him come out of the darkness into the light. He returns with the knowledge that he is capable of evil, but he won't allow it to control him. We never know what another person is truly thinking or what he/she is deep down in his/her soul, do we? That is the darkness/evil that lies in each of us. I guess the trip could be compared to hell, but those who go to hell aren't allowed to return, whereas Marlow is.
Oh, certainly! I connect this book with Dante's Inferno. In both tales, the narrator continues to encounter worse examples of human cruelty and corruption and both narrator's escape, having learned a lesson. Dante is walking through Hell itself - Marlow is walking through the Congo. But each major event shows the depths of evil in the soul to a larger degree. Marlow is lead by Kurtz, a Satan-like character who shows no soul and no sense of morality. Kurtz slowly seeps into Marlow and influences him; it is Kurtz's death that allows Marlow to break from the spell and recognize the spiritual danger he is in.