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"I had turned to the wilderness really, not to Mr. Kurtz, who, I was ready to admit, was as good as buried." (3.20)
Marlow seems to see Kurtz and the wilderness as different, but from where we are (safely on the other side of the page) they look pretty similar: dark, evil, and inescapable.
"I felt an intolerable weight oppressing my breast, the smell of the damp earth, the unseen presence of victorious corruption, and the darkness of an impenetrable night…" (3.20)
Check this out: once Marlow decides to go over to the dark side, the imagery starts getting pretty grave. Literally. A weight, damp earth, "corruption," darkness—doesn't it sound a little (okay, a lot) like he's being buried alive?
"He [the harlequin] informed me, lowering his voice, that it was Kurtz who had ordered the attack to be made on the steamer. 'He hated sometimes the idea of being taken away - and then again. . . ." (3.22)
Kurtz is so far gone that he actually orders an attack on the men sent to rescue him. There's literally no difference between black and white to him—but it's not exactly a coca-cola vision of racial harmony. It's more like a jungle nightmare.