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"I've been telling you what we said - repeating the phrases we pronounced—but what's the good? They were common everyday words - the familiar, vague sounds exchanged on every waking day of life. But what of that? They had behind them, to my mind, the terrific suggestiveness of words heard in dreams, of phrases spoken in nightmares." (3.29)
By this point, Marlow is basically living in a waking nightmare. He seems to be at a point where he can't even tell what's real.
"But his soul was mad. Being alone in the wilderness, it had looked within itself, and by heavens! I tell you, it had gone mad. I had—for my sins, I suppose—to go through the ordeal of looking into it myself. No eloquence could have been so withering to one's belief in mankind as his final burst of sincerity. He struggled with himself, too. I saw it—I heard it. I saw the inconceivable mystery of a soul that knew no restraint, no faith, and no fear, yet struggling blindly with itself." (3.29)
Marlow claims that being alone in the wilderness has made Kurtz crazy, but it's complicated: Kurtz knows he's crazy, but he doesn't know that it's his own lack of self-restraint—or maybe, lack of human restraint—that's made him that way. He can't get out. In fact, it seems like he's made himself mad.
"[…] I heard him mutter, 'Live rightly, die, die…' I listened. There was nothing more. Was he rehearsing some speech in his sleep, or was it a fragment of a phrase from some newspaper article? He had been writing for the papers and meant to do so against, 'for the furthering of my ideas. It's a duty.'" (3.39)
Kurtz just gets crazier and crazier as he dies, sputtering random words like a mid-grade hip-hop artist.