Marlow understands and depicts Kurtz as a monster who rips the veil off any pretense that Europeans are colonizing to do "good" or bring civilization to the African natives. Kurtz moves from good intentions to becoming so overtaken with a lust for profit from ivory that he allows the natives to worship him as a god and treats them with extreme cruelty. He brings in more ivory than his peers but also upsets his higher-ups because of his unvarnished rapacity. In doing so, Kurtz—in being unabashedly about profit— exposes the hypocrisy of the Europeans' stated humanitarian reasons for being in Africa.
"All Europe contributed to the making of Kurtz," Marlow says. The moral complexity lies in that it is impossible to condemn Kurtz without condemning European imperialism as a whole—and that is what few among Marlow's peers want to do. It is easier to condemn an individual than an entire system, to see the "bad" person as aberrant—but this will not do. As many critics have pointed out, Kurtz is "hollow:" in many he is the reflection of other people's desires or projections. The moral dilemma then becomes: who is responsible for this man? How do we undo a system that he exposes as evil that yet makes life so much more comfortable for those far away in Europe? Today we can see imperialism as evil: a cognate for our times might be the evils of a carbon-based energy system that makes life comfortable to many but at what price?
Marlow can see the evil, but he can't deny that Kurtz was also other things than a greedy madman. He painted, so was an artist, and in some ways Marlow also admires him for his grandeur in refusing to put up a facade to hide what he really was.
At the end of the novel, Marlow refuses to tell Kurtz's fiancee the reality of what life was like in the Congo or Kurtz's true last words. Instead, he tells her Kurtz's last words were her name. He justifies this to the reader by stating that he can't live if there are no people in the world (women) who believe the illusion that imperialism—and hence European civilization— is good. This brings up another moral question: who, in the end, is worse: Kurtz, who exposes the insane greed and cruelty of the European system or Marlow, who covers it up and hence enables it?