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"Mind, I am not trying to excuse or even explain - I am trying to account to myself for—for—Mr. Kurtz—for the shade of Mr. Kurtz. This initiated wraith from the back of Nowhere honoured me with its amazing confidence before it vanished altogether." (2.29)
Kurtz isn't a whole human being; he's a "shade" or "wraith," something that is literally a ghost of its former self and on the verge of vanishing into nothingness. It seems that he's completely lost his identity in the jungle.
"And the lofty frontal bone of Mr. Kurtz! They say the hair goes on growing sometimes, but this—ah—specimen, was impressively bald. The wilderness had patted him on the head, and, behold, it was like a ball—an ivory ball; it had caressed him, and—lo!—he had withered; it had taken him, loved him, embraced him, got into his veins, consumed his flesh, and sealed his soul to its own by the inconceivable ceremonies of some devilish initiation." (2.29)
This isn't your run-of-the-mill male pattern baldness: this is some crazy wilderness sorcery. Like his hair, Kurtz's flesh has been "consumed" and his soul has been "sealed"—cut off from the rest of humanity. Mr. Kurtz has lost both physical and spiritual aspects of a human being; he's no longer whole.
"[…] how can you imagine what particular region of the first ages a man's untrammeled feet may take him into the way of solitude—utter solitude without a policeman—by the way of silence—utter silence, where no warning voice of a kind neighbour can be heard by the whispering of public opinion?" (2.29)
Alone in the interior, isolation warps your mind. (The same might be said of sitting alone in front of a computer all day. Ahem.) After a few days of silent isolation, the men can't judge things properly.