The dichotomy between truth and falsehood is a huge theme in Heart of Darkness, and it largely revolves around the moral compass of the protagonist, Marlow. In the book, Marlow emphasizes his deep disdain for "the lie" as a concept. He maintains that the lie has a "flavor of mortality" and telling one is analogous to biting into rotten food.
Marlow tells two lies over the course of the story. One is an attempt to assist Kurtz, and the other is to sustain a positive image of Kurtz's memory after he dies.
In his first lie, he convinces the brick maker that he is a far more influential member of the company than he actually is. He does this to appear powerful in order to assist Kurtz—in an attempt to stop Kurtz's detractors from attacking him or even to remove him from the jungle. However, this attempt to help Kurtz is ultimately doomed.
The second lie Marlow tells is to the fiancé of the late Kurtz, who had waited two years for his return from Africa. Marlow cannot bring himself to reveal the true nature of what Kurtz had become and preserves Kutrz's image as a good man so that his fiancé can more easily accept his passing.
Even though these are lies told with the best of intentions, Marlow is still revolted by them. This is not because of the taint that they represent for his own morality—for Marlow explicitly states that his feelings about lies do not come from moral superiority—but because the darkness of the world forced him to obfuscate the truth for the greater good. The disgust he feels with these lies is similar to the disgust he feels concerning the lies that western society tells in order to rationalize the efforts of the company.