Is Heart of Darkness the story of society's corruption and exploitation or personal corruption and greed?

I don't like the way that your question seems to force you
into making a choice between these two options. Actually, I would say that the
overwhelming message of this excellent novella indicates that both answers are correct.
The story focuses just as much on the way that colonialism as a whole functions to
exploit and corrupt as it also looks at the individual in such an arena and examines how
character is decayed and eated away by occupying such an arena without law and order to
restrain the darker instincts of man.

To examine the way
that society as a whole is shown to be corrupt and exploitative, consider Marlow's
description of colonialism at the beginning of the
story:

It was
just robbery with violence, aggravated murder on a great scale, and men going at it
blind--as is very proper for those who tackle a darkness. The conquest of the earth,
which mostly means the taking it away from those who have a different complexion or
slightly flatter noses than ourselves, is not a pretty thing when you look into it too
much.

Clearly, describing the
entire colonial enterprise in these terms establishes the way that it was based on
society's corruption and exploitation. Yet, at the same time, equally the story focuses
on the way that personal corruption and greed can result in the downfall of noble
characters, namely Kurtz. Note how this is symbolised through the way that Kurtz chose
to have his hut decorated:

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Then I went carefully from post to post with my
glass, and I saw my mistake. these round knowbs were not ornamental but symbolic; they
were expressive and puzzling, striking and disturbing--food for thought and also for
vultures if there had been any looking down from the sky; but at all events for such
ants as were industrious enough to ascend the pole. They would have been even more
impressive, those heads on the stakes, if their faces had not been turned to the
house.

Kurtz's moral
corruption is indicated through the abominable crimes he committed in his quest for
gaining ever greater quantities of ivory and the way that this hunger conquered his
moral scruples. This novel therefore clearly indicates the way in which colonialism
features the corruption of society and individuals.