At the beginning of the action in the novella, Marlow, the narrator, is confused about Kurtz and his motives. After he first arrives in Africa, he hears people who work for the ivory company tell a story in which Kurtz returns to his station, though it is bare of food. Marlow thinks of Kurtz: "I did not know the motive. Perhaps he was just simply a fine fellow who stuck to his work for its own sake" (page numbers vary by edition). At this point, Marlow believes that Kurtz is benevolent.
As he journeys towards Kurtz's inner station, Marlow finds Kurtz intriguing and puzzling. He comes upon a report Kurtz has written for the International Society for the Suppression of Savage Customs. At the end of this report, which is eloquent and altruistic towards the native Africans, Kurtz adds, "Exterminate all the brutes!" Marlow compares this statement to "a flash of lightning in a serene sky." At this point, Marlow is puzzled about why Kurtz would add this postscript to a report in which he has advocated benevolent treatment of African people.
In the end, Marlow knows that Kurtz is flawed and has elevated himself about the native people by pretending to be a deity. In the end, however, Marlow has undying dedication to Kurtz. As he says,
"He had summed up -- he had judged. `The horror!' He was a remarkable man. After all, this was the expression of some sort of belief; it had candour, it had conviction, it had a vibrating note of revolt in its whisper, it had the appalling face of a glimpsed truth -- the strange commingling of desire and hate" (page numbers vary by edition).
Marlow remains dedicated to Kurtz because he believes that Kurtz, whose last words were "the horror," understands the truth of what European colonization in Africa is truly like. He finds Kurtz "remarkable" and cannot shake his admiration of him, even though he knows that Kurtz in reality was far from perfect.
Marlow's feelings for Kurtz change greatly over the course of the novella. In the beginning Marlow admires and respects Kurtz for his accomplishments. The, as Marlow steams up the Congo and he hears more of how Kurtz has managed to accomplish his feats, he questions his own admiration and respect for the man. Finally, once they are face to face Marlow's respect is replaced by fear for that the man represents and the evil that he displays. In my mind, it is Marlow's description and reflection upon discovering the human heads placed on sticks by Kurtz that represents the moment whatever flickers of respect he holds completely disappear.