This brilliant novella touches upon many themes, but principally, the overarching concern of this novel is the state of man's soul and moral corruption. This, in a sense, is what all the other themes of this story lead to, much as different tributaries of a river eventually lead back to one source. Above all, the insanity, racism, violence and solitude depicted in the novel find their source in the character of Kurtz and his acts of reckless, insane violence in the "heart of darkness." Again and again, Kurtz is shown to be a character who has exchanged reason for insanity. It is this exchange that has allowed his most brutal and base instincts to rule his actions. The character of Kurtz shows the dangers of giving oneself over to one's evil instincts and leadings. The degradation of Kurtz is not only a result of his solitude and isolation, but also, Conrad suggests, of the inherent nature of man's soul and what it means to be human. Conrad presents a grim view of the world, as he indicates his belief that in all humans exists the capacity for becoming truly evil. Kurtz is given a moment of insight when, reflecting upon his own soul and its descent, he utters the famous words, "The horror! The horror!" The lesson is not lost on Marlow, who realises the universal nature of Kurtz's descent. He, too, living in a state without the boundaries of society to maintain his reason, has the potential to descend into such an anarchic state.
Note the way that Marlow tries to describe to his listeners the importance of the role of civilisation in restraining such baser instincts:
You can't understand. How could you? With solid pavement under your feet, surrounded by kind neighbours ready to cheer you or to fall on you, stepping delicately between the butcher and the policeman, in the holy terror of scandal and gallows and lunatic asylums--how can you imagine what particular region of the first ages a man's untrammeled feet may take him into by the way of solitude--utter solitude without a policeman--by the way of silence--utter silence, where no warning voice of a kind neighbour can be heard whispering of public opinion?
The overarching concern of this novel then is the heart of darkness that lies within us all, in the form of man's innate capacity for evil that emerges if we exist in a place where there are no restraining forces of civilisation in the form of the "holy terror of scandal and gallows and lunatic asyllums" ready to restrain such baser instincts.