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"But the wilderness had found him (Kurtz) out early, and had taken on him a terrible vengeance for the fantastic invasion. I think it had whispered to him things about himself which he did not know, things of which he had no conception till he took counsel with this great solitude - and the whisper had proved irresistibly fascinating." (3.5)
Check out how Marlow personifies the wilderness, making it into a living, breathing force of evil.
"His [Kurtz's] ascendancy was extraordinary. The camps of these people surrounded the place, and the chiefs came every day to see him. They would crawl. '[…] I don't want to know anything of the ceremonies used when approaching Mr. Kurtz,' I shouted. Curious, this feeling that came over me that such details would be more intolerable than those heads drying on the stakes under Mr. Kurtz's windows. After all, that was only a savage sight, while I seemed at one bound to have been transported into some lightless region of subtle horrors, where pure, uncomplicated savagery was a positive relief, being something that had a right to exist—obviously - in the sunshine." (3.6)
Marlow still has a shred of morality left—let's say, enough to keep from downloading illegal music, but not enough to keep from sharing his Netflix login. He's not horrified at the thought of living in a world where evil can exist openly, but he is terrified by the thought of people (like the native Africans) openly worshipping evil.
"My hour of favour was over; I found myself lumped along with Kurtz as a partisan of methods for which the time was not ripe: I was unsound! Ah! but it was something to have at least a choice of nightmares." (3.19)
Heading into the interior teaches Marlow that there's really no such thing as good or evil: there's only evil and slightly less evil.