Marlow’s journey toward the interior and toward Kurtz seems inevitable, as if Marlow is drawn nearer and nearer to the heart of darkness by his own morbid curiosity and by his childhood drive to explore. Indeed, the two women knitting in Brussels represent the Fates of ancient Greek mythology. With their appearance, Marlow begins to feel as if his journey is ill-starred – yet he forges on anyway. The interplay between fate and free will informs the action of the plot, calling into question whether Marlow could have avoided his descent into madness, his corruption, and his subsequent revelations about human nature.
Marlow cannot help but meet Kurtz: he is destined to go into the interior, experience it much as Kurtz did, and eventually meet the man himself. If we accept Kurtz as Marlow’s foil, this means that Kurtz was fated to go mad in the interior and couldn't stop it by any conscious decision.
Marlow’s meeting and renunciation of Kurtz is a result of personal choice; in other words, he could have glorified Kurtz as the others did, but he made the choice not to.