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"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.
"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?