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"But the wilderness had found him 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. It echoed loudly within him because he was hollow at the core […]." (3.5)
The evil wilderness drains Kurtz of his heart and his humanity, leaving him "hollow at the core." Brain snack: the idea that we're all "hollow" at the core and that there's no clear meaning is basically the major idea of early twentieth-century writing. T.S. Eliot wrote a whole poem about it.
"I was struck by the fire of his eyes and the composed languor of his expression. It was not so much the exhaustion of disease. He did not seem in pain. This shadow looked satiated and calm, as though for the moment it had had its fill of all the emotions." (3.10)
Kurtz isn't even a whole human any more: he's a "shadow." Pros: because he's no longer fully human, Kurtz doesn't feel the pain of his disease as his body wastes away; he seems calm in a wholly inhuman way. Cons: He's, um, not fully human.
"And from right to left along the lighted shore moved a wild and gorgeous apparition of a woman." (3.13)
The fact that this woman is described as an "apparition" suggests that Marlow does not consider women, especially this native African one, to be as fully human or as capable as men. Similar language comes up with the Intended shows up at the end of the novel—not the "wild" bit, but the "apparition" part.