What moment is Marlow referring to and what does he mean by complete knowledge?
When Marlow asks if Kurtz lived his life again in every detail of desire, temptation, and surrender during that supreme moment of complete knowledge...
Marlow here refers to the moment just before Kurtz' death. At this moment, Marlow reports a "change that came over [Kurtz'] features" that he had "never seen before." What exactly Kurtz saw at that moment is ambiguous. All that we know for certain is that Kurtz' vision caused him to spontaneously cry out: "The horror! The horror!" The context leads us to believe that Marlow thinks that Kurtz' life was flashing before his eyes, and that he finally came to terms with his terrible deeds. Therefore, his last words could be read as "a judgment upon the adventures of his soul on this earth." This is the "complete knowledge" with which (Marlow feels) Kurtz must come to terms.