Many people seem to think that this story is just about racism, but that is missing the main point. It is true that much of Conrad's fiction seems racist in tone, but one must take that from whence it comes; he was writing at a time when European Colonialism, (and sadly racism too) was in full swing. It is of course inevitable that writers will reflect some of the mores of their era, and also that some writers will examine the prevailing mores and comment on them.
However, the inner message of the story transcends dealing with just purely the manifestation of racism and colonial exploitation, although such exploitation does of course also play a role in the density of ideas, and, on the surface, forms the main theme of this novel.
But the inner, integral theme has to do with the more transcendental issue of how wordly power corrupts the holder thereof; about the inner boundaries set by conscience, and the comfort it brings to remain within those boundaries. Conversely, what happens to your psyche when one crosses these boundaries and enters an area beyond what you were brought up to believe fell within acceptable behavior?
I see Conrad exploring the territory beyond those boundaries, about what happens when an individual crosses the boundaries set by conscience and social conditioning just because he finds himself in circumstances where he can cross these boundaries.
Parallels for such circumstances can be seen in the excesses certain Roman emperors indulged in, simply because they had the power to. They held sway over the life or death of countless individuals, and many of them indulged in this power to excess.
However, Conrad uses a fresh setting in which to explore the issue, and it is a setting that is more intimate and personal, and just as disturbing.