The book holds a somewhat pessimistic view of human nature. It suggests that, despite surface appearances, human nature remains overwhelmingly subject to instinctual passions, emotions, and appetites – that essentially, it remains primitive. These primitive impulses are fully awakened in people like Kurtz in the midst of Africa, the so-called Dark Continent. In the general discourse of the age in which this novella was written, Africa was viewed very much as a hotbed of primitivism and savagery. However, it is the morally corrupt, materialistic, greedy Europeans who rush to plunder Africa's people and its natural resources that are seen as being the worst of all in this story.
Marlow issues a sobering reminder at the very start of the book that London, the heart of one of the supposedly most modern and advanced civilisations, is not necessarily as refined as it seems, or certainly has not been in the past:
'And this also,' said Marlow suddenly, 'has been one of the dark places of the earth.'
Therefore London, and not Africa, is the first place to be linked with the idea of darkness, and the suggestion of darkness returns at the end of the novel, overspreading the Thames. The behaviour of the Europeans in Africa certainly seems to confirm the idea that they are really no more civilised than the natives whom they oppress.
The book gives a picture of human nature as being as vast and deep as any continent. This is apparent, for example, when Marlow finds himself responding to the primitive dancing and singing among the natives. He feels the primitive tug in himself, because, as he says:
The mind of man is capable of anything - because everything is in it, all the past as well as all the future.
In other words, human nature is seen to be all encompassing. It can take in at least the veneer of civilised sophistication while still retaining primal impulses. Sober, relatively well-balanced individuals like Marlow can become aware of this duality without giving way to extreme emotions, but such an awareness is seen to have a terrible effect on a person like Kurtz, who is creative, passionate, idealistic – in a word, unstable.
Interestingly, the novella also hints at a contrary view to this idea of human nature as being wide and deep, in the depiction of the general manager and the brickmaker. These characters appear to be empty vessels, devoid of any real feeling. Marlow speculates, for instance, that the reason the manager is never ill is because ‘there was nothing in him’, and the brickmaker is memorably described as being ‘papier-mache’:
It seemed to me that if I tried I could poke my finger through him, and would find nothing inside but a little loose dirt, maybe.
This man, then, seems to be composed of no real substance at all. The degenerate darkness inside Kurtz is terrifying, but the sheer emptiness which is seen to inhabit the manager and the brickmaker is maybe even more chilling.
In Heart of Darkness, Marlow repeatedly refers to virtue and civilized behavior as a constructed thing, telling his captive audience that they could never truly understand the actions of the people stationed in the Congo because they still lived where no one had to be alone with their own human nature and were instead able to rely on the outward restrictions placed upon that nature by their environment. This attitude is illustrated well in Marlow's address to his crew after telling them about Kurtz's inner darkness.
"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 untrammelled 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 neighbor can be heard whispering of public opinion. These little things make all the great difference. When they are gone you must fall back on your own innate strength, upon your own capacity for faithfulness."
It is only because his crew are used to living in a world of enforced order that they could not contemplate going down the same road Kurtz has walked. The butcher must face animals and get blood on his hands, but nothing but conquered remains reach the people. The policeman can be relied upon to fight all of their battles while they stay safe in their homes. There is no call to fend for themselves or rely on their own strengths in a place where there are all of these restrictions on their actions. Without these trappings, they would be alone and forced to resist temptations and ignore the savagery in their natures based on their own merits.