In Heart of Darkness, how is Marlow's ironic rendering of an experience he had 20 years earlier a means to make amends for his immature attitudes?

At the beginning of the novella "Heart of Darkness," Marlow, now in England, meditates about what it might have been like for a Roman soldier to have journeyed to current-day England. He describes the experience as "utter savagery...all that mysterious life of the wilderness that stirs in the forest, in the jungles, in the hearts of wild men" (page numbers vary by edition). In other words, the soldier would have been thrown into savage conditions, and, as Marlow says, "There's no initiation either into such mysteries. He has to live in the midst of the incomprehensible, which is also detestable" (page numbers vary by edition). Marlow is, of course, describing his own experience in the Congo, when he was thrown into a situation of barbarity without any preparation. 

Now, looking back at his idolization of the evil Kurtz, Marlow feels some remorse. He now knows that colonization only leads to evil. As he says:

"The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses than ourselves, is not a pretty thing when you look into it too much. What redeems it is the idea only" (page numbers vary by edition).

In retelling his story, including his idealization of Kurtz and his meeting with Kurtz, Marlow recounts the evils of European colonization in Africa and the ways in which Kurtz and the other Europeans brutalized the people of the Congo. He tells his story with the wisdom of age and criticizes his own idealism as a youth. His story is filled with irony, as the way he pictured Kurtz and the Congo was totally different than their stark reality, and his retelling of this story is a way to correct his own misunderstandings in the past.