As Kurtz nears death, Marlow finds himself more and more fascinated by what Kurtz has to say, and by his past in the jungle. Marlow believes that the jungle has destroyed whatever was truly human about Kurtz, and that Kurtz is well aware of that loss. When he is dying, Kurtz seems to both fear and accept his death:
"One evening coming in with a candle I was startled to hear him say a little tremulously, 'I am lying here in the dark waiting for death.'"
He cried in a whisper at some image, at some vision -- he cried out twice, a cry that was no more than a breath--
"'The horror! The horror!'"
Marlow believes that at the last, Kurtz regained a measure of his humanity and fully understood how his actions were harmful and inhuman. Marlow also thinks that Kurtz's domination of the jungle -- in opposition to its subjugation of his morality -- is a sign of how truly powerful Kurtz was.
...I understand better the meaning of his stare... piercing enough to penetrate all the hearts that beat in the darkness. He had summed up -- he had judged. 'The horror!'
(Conrad, Heart of Darkness, eNotes eText)
The others on the ship, the slaves and the Company men, have no interest in Kurtz as anything other than an end to their goals. The Company men in particular are pleased that Kurtz has died, and have no understanding of his epiphanies or Marlow's kinship. Marlow, in almost succumbing to the same psychological fate as Kurtz, treats Kurtz's last words as the man's personal epitaph, his summation of everything he has seen and done, and as his personal judgement on the world around him. If Kurtz has committed horrors, Marlow believes, it is because the jungle contained horrors of its own, and faced with the choice of becoming a worse horror in himself or being consumed, Kurtz chose the horror.