I believe the title of Heart of Darkness refers to humanity and their natural instincts. Throughout the story, Marlow is trying to reveal the true nature of man and finds that everyone has a dark side. No matter how hard humans try, there is always something that makes the soul essentially dark. Take for instance Fresleven from part one, he was “the gentlest, quietest creature that ever walked on two legs.” Despite this, he still ended up attacking the chief of the village. It also refers to how greed can overcome people, and make them do terrible things. Kurtz is so obsessed with ivory and money, that he is willing to take out anyone who gets in the way of it. Kurtz only understands his wrongdoings on his deathbed, and is finally being aware of all the sins he has committed.