In Heart of Darkness, what is Marlow's purpose in telling his story to the others?
Marlow is depicted as "a meditating Buddha" who shares his enlightening experiences with the other passengers of the Nellie as a therapeutic exercise, as well as an opportunity to commentate on the negatives of imperialism and humanity's inherent wickedness. Marlow begins by commenting on the nature of civilization after contemplating the Romans' initial thoughts as they traveled up the Thames River for the first time to "civilize" the savages in Britain. Marlow then says,
"I don’t want to bother you much with what happened to me personally . . . yet to understand the effect of it on me you ought to know how I got out there, what I saw, how I went up that river to the place where I first met the poor chap." (3)
Marlow's comment is significant and offers insight into his motivation to tell his story. Marlow clearly has been dramatically impacted by his experiences in the Congolese jungle and has been carrying the weight of his story for some time. The ensuing trip offers Marlow an opportunity to tell his story to others as a therapeutic release. Throughout Marlow's story, he comments on the inefficiency, greed, and true nature of imperialism.
His trip into the African continent also gives him insight into the "darkness" of the human heart. Upon meeting Kurtz, Marlow perceives the unrestrained wickedness that resides in every human. His shocking, enlightening experience has dramatically affected his view of humanity, imperialism, and civilization. Marlow's commentary aboard the Nellie provides him with an opportunity to therapeutically convey his difficult experiences while simultaneously commentating on the negative aspects of humanity and society.
Marlow has experienced a vast change in his character, and his contemplation of the similar effects on the Romans who might have landed on British shores spurs him to tell his story to the others on the boat. He might be telling the story for the first time, and as he gets more involved in the tale it becomes clear that he is unburdening himself, explaining his motivations and justifying his actions in a way that allows him some level of relief from the years of carrying these memories. He starts by saying:
"I don't want to bother you much with what happened to me personally..."
This is perhaps because he originally intends only to tell some of the minor details and superficial aspects, and thus tie it back to his observations on the "darkest places" of the Earth. However, as the trip goes on and nobody stops him, Marlow becomes an orator, an old-fashioned storyteller, reciting by memory an extraordinary experience that truly did change him. At the end, Marlow states:
"Hadn't he said he wanted only justice? But I couldn't. I could not tell her. It would have been too dark -- too dark altogether...."
(Conrad, Heart of Darkness, gutenberg.org)
This is in itself both his justification for hiding Kurtz's true evil from The Intended, and also for his own silence; Marlow is barely able to accept the story himself, but sharing it separates him from it to the extent where he can finally accept how truly "dark" the story is, and how deeply it rooted itself into his inner being.