The reader can assume that Marlow is experienced in the ways of the world: his career as a ship's captain would indicate that he is able to handle himself on a ship,...
especially in that sailing could be treacherous and deadly in the blink of an eye.
However, as the story progresses, Marlow's rational mind can hardly comprehend what he sees. Arriving at the Lower Station, a road is being constructed on a cliff, but the blasting of dynamite there serves no logical purpose: the men working seem crazy to Marlow.
A heavy and dull detonation shook the ground, a puff of smoke came out of the cliff, and that was all. No change appeared on the face of the rock...the cliff was not in the way or anything...
Next he sees machinery (we could infer that it is functional) just lying around on the ground rusting and deteriorating.
...an undersized railway truck lying there on its back with its wheels in the air. One was off. The thing looked as dead as the carcass of some animal.
However, what opens Marlow's eyes dramatically is his witness of how horrifically the natives are treated. They are shackled like animals and there is a look of death in their eyes. Their bones and joints protrude.
They passed me within six inches...with [a] complete death-like indifference...
They were dying slowly—it was very clear.
Death here is commonplace. What Marlow sees he cannot countenance—word of these kinds of conditions have not reached "civilization," and he is unprepared for the horror he encounters.
While these things provide enlightenment regarding the ivory trade, nothing could have prepared Marlow for what remains of the brilliant and impressive Kurtz: he has been greatly successful in exporting more ivory than all the other agents combined.
Hadn't I been told in all the tones of jealousy and admiration that he had collected, bartered, swindled, or stolen more ivory than all the other agents together?
The madness, the drive for money, has destroyed Kurtz.
[Kurtz] is revealed upon acquaintance to be a dying, deranged, and power-mad subjugator of the African natives. Human sacrifices have been made to him.
He has not left the jungle in over a year: he is worshiped like a god, and the building Marlow first sees when they arrive is surrounded by pikes with heads on the top of each.
When Marlow finally meets Kurtz, the other man frightens and amazes Marlow:
Anything approaching the change that came over his features I have never seen before, and hope never to see again...I saw on that ivory face the expression of somber pride, of ruthless power, of craven terror—of an intense and hopeless despair.
Marlow has heard of Kurtz's capacity for greatness, but now he witnesses man's capacity for human desolation and corruption. The once-brilliant Kurtz is gone; he is waiting—he tells Marlow—to die. Marlow tries to take Kurtz away—but Kurtz does not get far before he does, in fact, die.
Marlow has learned the cost of amassing a fortune. He has discovered that the quest for riches can rob a man not only of his moral fiber, but also of his humanity and his mind—as seen with Kurtz. Marlow has learned that the truth of the ivory trade is a well-kept secret, and ironically, he carries on the lie at the end, finding it impossible to cause Kurtz's fiancee ("intended") any more pain with the truth of the man—the stranger?—she loved so deeply.