“This man suffered too much. He hated all this, and somehow he couldn’t get away.”
Well before the time Marlow reaches Kurtz in the Inner Station, Kurtz had been doomed to suffer there in response to everything he had done to locals and to his own heart. Being isolated from the rest of civilization, he was left to eventually become rampant and in touch with the evils of greed, corruption and loneliness. When he originally arrived in The Congo, as shown in the opening segment of his manuscript, his goals were not outwardly evil, he insisted that his presence could be a force for "unbounded good" (61).
Over time, Kurtz was subjected to the horrors of the unknown land and the avarice in the ivory hunting business, thus causing him to revise his purpose for being there to a more accurate description of his actions; he came to "exterminate all the brutes" (62). From there, Kurtz's health begins to deteriorate in intervals, and his soul becomes more warped with each sickness. This culminates to where he can no longer function on his own and when he decides to return home with Marlow. This came too late for Kurtz, however, as upon realizing the depth of his distortion, of his crimes against other humans, he truly understands "the horror" that he has captured himself in (92).
"''The last word he pronounced was-- your name.' 'I heard a light sigh, and then my heart stood still, stopped dead short by an exulting and terrible cry, by the cry of inconceivable triumph and of unspeakable pain...The heavens do not fall for such a trifle. Would they have fallen. I wonder, if I had rendered Kurtz that justice which was his due? Hadn't he said he wanted only justice? But I couldn't. I could not tell her. It would have been too dark-- too dark altogether...'" (96).
As Marlow lies to the intended about her fiancee's final words, it is made clear that Marlow was more concerned with protecting the withstanding image of Kurtz that the woman had, as it was idealized and contained admiration and wonder, similar to what Marlow had felt about Kurtz before meeting him. With that, it can be inferred that, despite Kurtz wanting justice for his actions in the end, Marlow denied him that by refusing to expose the darkness of the idol to the disillusioned intended, as that would have been "too dark altogether" (96). Some part of Marlow wanted to remain faithful to the idea that Kurtz wasn't completely monstrous, that he still had some aspects of honor within him.
Moreover, this restraint that Marlow shows toward revealing the real image of Kurtz appears before speaking with the Intended, for it occurs when Marlow speaks with the Company representative and Kurtz's cousin. As the two men come to reclaim any documentation of Kurtz's voyage, Marlow offers them the copy of the "Suppression of Savage Customs" Kurtz had claimed, stripped of the post script containing the condemning evidence of his madness (89). This gesture shows that Marlow is protective of the image of Kurtz, moreso that the darkness of Kurtz's heart may not be illuminated to anyone but himself, in that no one would understand Kurtz as Marlow did.