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"He [Kurtz] rose, unsteady, long, pale, indistinct, like a vapour exhaled by the earth, and swayed slightly, misty and silent before me […]." (3.27)
Kurtz is a "vapour," "misty" and insubstantial before the wholly human form of Marlow. Pro tip: eat more protein, Kurtz!
"He [the manager] leaned back, serene, with that peculiar smile of his sealing the unexpressed depths of his meanness." (3.44)
The manager may have feelings about Kurtz's death (we doubt it), but we'll never know: the depths of his "meanness" are locked up by his vacant smile. As expected, he shows no real emotion at the news.
"I have wrestled with death. It's the most unexciting contest you can imagine. It takes place in an impalpable greyness, with nothing underfoot, with nothing around, without spectators, without clamour, without glory, without the great desire of victory, without the great fear of defeat, in a sickly atmosphere of tepid skepticism, without much belief in your own right, and still less in that of your adversary." (3.48)
The dominant feature of Marlow's struggle with death is its emptiness. It takes place without anything underneath or around it, without the possibility of human contact, without noise or glory, without the desire to win or fear of losing. Most tragically, it's fought without conviction in one's own beliefs. It's as empty and meaningless a struggle as can be.