2nd Heart of Darkness Prompt

Per Mr. Duncan...

Lying & dying

Part 1 ¶ 62:
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.

What lie does Marlow tell for Kurtz, and why does he tell it? How does Marlow become entangled in a lie--bite into something rotten--when he signs on as a steamboat captain in the Congo?