You are correct in saying that Marlow is the character who speaks about detesting a lie. The following excerpt is from Section I, of Heart of Darkness by Joseph Conrad:
I would not have gone so far as to fight for Kurtz, but I went for him near enough to a lie. You know I hate, detest, and can’t bear a lie, not because I am straighter than the rest of us, but simply because it appalls me. There is a taint of death, a flavour of mortality in lies—which is exactly what I hate and detest in the world—what I want to forget. It makes me miserable and sick, like biting something rotten would do. Temperament, I suppose.
Here, Marlow is speaking. Even if one is not familiar with the text, the fact that the speaker names Kurtz in the excerpt proves that it cannot be Kurtz speaking. The quote is important given Marlow is being very blatant about how he feels about lies (as told by the unnamed narrator, given Marlow is not the narrator). Not only does Marlow detest lies, lies make him ill. He passes off how he feels about lies as being part of his temperament alone.