Marlowe is a good storyteller, or at least the narrator’s rendition of Marlowe is a good storyteller—it’s important to remember that all of Marlowe’s words reach us second hand. Part of the theme of the story is that wildness lurks everywhere in the world; Marlowe begins by saying that “this also” (meaning the Thames) “has been one of the dark places of the earth.” It is a matter of perception: most people would look at the Thames and see civilization, but Marlowe knows better.
Conrad reinforces this notion using point of view. In crucial moments of the story, the language limits our perception. Take, for instance, the moment when his pilot is killed:
Looking past that mad helmsman, who was shaking the empty rifle and yelling at the shore, I saw vague forms of men running bent double, leaping, gliding, distinct, incomplete, evanescent. Something big appeared in the air before the shutter, the rifle went overboard, and the man stepped back swiftly, looked at me over his shoulder in an extraordinary, profound, familiar manner, and fell upon my feet. The side of his head hit the wheel twice, and the end of what appeared a long cane clattered round and knocked over a little camp-stool. It looked as though after wrenching that thing from somebody ashore he had lost his balance in the effort. The thin smoke had blown away, we were clear of the snag, and looking ahead I could see that in another hundred yards or so I would be free to sheer off, away from the bank; but my feet felt so very warm and wet that I had to look down. The man had rolled on his back and stared straight up at me; both his hands clutched that cane. It was the shaft of a spear that, either thrown or lunged through the opening, had caught him in the side, just below the ribs; the blade had gone in out of sight, after making a frightful gash; my shoes were full; a pool of blood lay very still, gleaming dark-red under the wheel; his eyes shone with an amazing lustre.
Here, we share Marlowe’s point of view. With him, we see the “vague forms” of men running in the bush; we don’t know who these people are or what they are doing. “Something big appeared in the air,” but, with Marlowe, we don’t know what this thing is; he is reporting his perception, which was just to register an brief impression of “something,” neither good nor bad. Marlowe continues reporting just the facts: “the rifle went overboard,” the pilot looks at him in a “profound, familiar manner,” his head hits the wheel not once but twice. He sees “what appeared” to be “a long cane” knocking over a camp stool. It is only after he notices that his feet “felt so very warm and wet” that these details coalesce into the shocking truth of the pilot’s death from a thrown spear. It is a moment when the mundane is suddenly turned into the horrific.
I don’t think Conrad’s point in Heart of Darkness is to argue that the Congo is an alien, savage place, not fit for civilized people. I think the point is that savagery is always everywhere just underneath the surface, and that the notion of “civilization,” like Marlowe’s first impression that the spear is a cane is simply a delusion, or a misinterpretation, of the way things really are.