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"I looked around, and I don't know why, but I assure you that never, never before, did this land, this river, this jungle, the very arch of this blazing sky, appear to me so hopeless and so dark, so impenetrable to human thought, so pitiless to human weakness." (3.3)
Nature seems to Marlow completely "hopeless" and "dark," entirely inaccessible to the human mind, incomprehensible and merciless to human weakness. You know, evil.
"Evidently the appetite for more ivory had got the better of the - what shall I say? - less material aspirations. However he had got much worse suddenly. 'I heard he was lying helpless, and so I came up - took my chance,' said the Russian. 'Oh, he's bad, very bad.'" (3.4)
Kurtz is a sick, sick man—in both senses of the word. He's physically sick, but he's mentally sick as well. In fact, it seems like we're supposed to identify physical sickness with mental sickness, as though having a migraine means that you're depraved.
"These round knobs were not ornamental but symbolic; they were expressive and puzzling, striking and disturbing - food for thought and also for vultures if there had been any looking down from the sky; but at all events for such ants as were industrious enough to ascend the pole. They would have been even more impressive, those heads on the stakes, if their faces had not been turned to the house. Only one, the first I had made out, was facing my way. I was not so shocked as you may think. The start back I had given was really nothing but a movement of surprise. I had expected to see a knob of wood there, you know. I returned deliberately to the first I had seen - and there it was, black, dried, sunken, with closed eyelids - a head that seemed to sleep at the top of that pole, and, with the shrunken dry lips showing a narrow white line of the teeth, was smiling, too, smiling continuously at some endless and jocose dream of that eternal slumber." (3.4)
Marlow's shocked to find out that those ornamental knobs surrounding Kurtz's house aren't wooden but, er, human. Yep, we're going to go with evil.