There is a definite sense in which Marlow changes dramatically as a result of his experiences with Kurtz and what he learns and witnesses. In one sense, Marlow, by the end of the story, seems to have much in common with the ancient mariner in Coleridge's "Rime of the Ancient Mariner," in the way that he seems fated to talk about and share what has happened to him. Note how Marlow describes his feelings and attitudes towards life after he returns to Europe from Africa, finding himself once again in the "sepulchral city" which he went to before accepting his commission, with people who continue to "dream their insignificant dreams":
They treaspassed upon my thoughts. They were intruders whose knowledge of life was to me an irritating pretence, because I felt so sure they could not possibly know the things I knew... I had no particular desire to enlighten them, but I had some difficulty in restraining myself from laughing in their faces, so full of stupid importance.
Marlow's character therefore irrevocably changes because of the "heart of darkness" that he witnesses in the character of Kurtz, and this is of course strengthened by the recognition that this "heart of darkness" is in all humans. Marlow glimpses a hidden truth, and, to a certain extent, he is unable to re-enter normal life once again in the same way precisely because, having seen the truth about the human condition, he cannot participate in the same "meaningless" activity that he sees everybody else engaging in.