Issues of race and imperialism dominate Conrad's Heart of Darkness. The work as a whole has been interpreted by successive generations of scholars and literary critics as providing the reader with a powerful critique of imperialism and all that it entails.
Imperialism, which Marlow describes in the story as “rapacious and pitiless folly,” is based on the idea that the white race is superior to all others, and therefore has the right to lord it over them. This idea was almost universally shared by white people in Conrad's day, which makes it all the more remarkable that he should take aim at the appalling treatment meted out to the Congolese people by their inhuman imperial overlords.
Even so, Conrad's approach to race and imperialism is not quite as straightforward as we might think. His portrayal of Africans has been described in some quarters as, at best, one-dimensional, and, at worst, racist. There's certainly no doubt whatsoever that Conrad's representation of Africans would not be considered acceptable in this day and age.
However, some critics have leaped to Conrad's defense by saying that the portrayal of Africans in the novel is exactly what one would expect to see through the eyes of a white colonialist like Marlow. One should never forget that what we see of Africa in the story is from Marlow's somewhat narrow perspective, so it's not surprising if Africans are portrayed as they are.
This in no way makes their portrayal any less offensive, but it does at least enable us to see that Conrad's treatment of the issues of race and colonialism, while being complex in other respects, is much less so in others.