Heart of Darkness might be a book about what happens when you let go of moral constructs and enter into the chaos of a self-made world.
I think we can read the character of Kurtz as a man who has attempted to go beyond "human morality" and in this attempt he fails. He is separated from the rest of mankind in this effort. He finds only "horror" and death, without a friend remaining to him.
Maybe we can say that Heart of Darkness is about morality as a unifying set of ideas. Morality connects people. Perhaps it is morality that functions as the glue of a society, however corrupt that society might be.
Even the most greedy of imperialists and ivory traders cannot understand Kurtz in the end, despite their willingness to exploit and pillage resources. They maintain some kind of morality, where Kurtz has eschewed all rules and laws of morality.
Many people believe that this book is a commentary on racism, but I think we can say that it could and should be taken further than that. The most important quote, I think, that makes this transition and connection possible is this:
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.
Here, Conrad combines the "conquest of the earth," which is the idea of imperialism, with the fact that conquests are completed against "weaker" societies. I say weaker in that they are less advanced in terms of weapons. They cannot defend themselves against oppression, most likely because they never have, and shouldn't have to.
Morally, Conrad illustrates this idea through his descriptions of the treatment of the natives. He describes them as faceless monsters, with "flailing limbs." This is where the racism aspect usually comes into play.
If we take it further, however, we can see that Conrad is critical of the greed that the imperialists bring. This is best illustrated through Kurtz, who through this lens, succumbs to his greed for ivory. He has put himself in this position, but the company has also encouraged him to push himself to the limit. Kurtz winds up over the limit, and at the end, we can interpret his famous words "the horror, the horror," as a cry of grief over what has become of him.