We follow Marlow’s story for the majority of the book, although he is not always our narrator. We sympathize with him because we assume he has a higher standard of morality than the majority of his colleagues. His motivation for going up the Congo River is not unadulterated greed or ambition for power. We know from the very beginning that he travels because he has a deep fascination with maps and unexplored spaces. He is also a good deal more sensible than his peers (such as the manager or brickmaker).
We realize as the plot continues that Marlow’s driving characteristic is his curiosity. From the very beginning, he is fascinated by Mr. Kurtz and as he is fed more and more information – sometimes through unscrupulous means – Marlow finds himself irresistibly drawn to the man.
When he reaches the interior – one of many hearts of darkness – Marlow finds himself at a moral dead end. He must make a choice between two evils and, surprisingly, siding with Kurtz seems the lesser of two evils. In making this "choice between nightmares," Marlow finds the fortitude to separate himself from his corrupted peers, though it is questionable how much virtue he saves by supporting heads-on-sticks Kurtz.
Marlow’s last vestige of illusion that the universe is fundamentally good is wiped away in his final interview with Kurtz’s Intended. After her naïve assessment of a situation which she never saw and, Marlow knows, was fundamentally different than she believes, Marlow lies to her. In some ways, this is about Marlow giving up. Taking on a pessimistic view of life, he realizes that the world is dark and he’d better just go along and lie about it than try to maintain his integrity by standing up to the darkness.