In Joseph Conrad's Heart of Darkness, I do not find any sense that Marlow is condescending toward the Africans. If anything, he is appalled at how the Europeans treat the natives. If he has difficulty with anyone, it is with the agents of the Company who see themselves as a vastly superior race, while exhibiting a total disregard for the population they have enslaved.
Marlow has been delivered into a part of the world where...
A prevalent feeling among Europeans...was that the African peoples required introduction to European culture and technology in order to become more evolved.
"Someone had to do it," was the attitude, and the term "white man's burden" shows just how superior Europeans saw themselves by comparison to the indigenous people of the Congo. King Leopold II of Belgium decided there was great wealth to be had in Africa—so he established a colony there, and his agents "raped" the land and its people.
Marlow is new to this part of the world, but it does not take long for him be horrified by what he sees:
Revulsion grows within him over the white man's dehumanizing colonization of the Congo.
Marlow witnesses the treatment of enslaved men, chained like animals...
Six black men advanced in a file, toiling up the path...I could see every rib, the joints of their limbs were like knots in a rope; each had an iron collar on his neck...
Nearby, Marlow finds another group of men. Some, he feels, have come to this spot simply to die. While the Europeans act as if the natives are a threat. Marlow sees none of this, and he is repulsed by how they are abused:
They were dying slowly—it was very clear. They were not enemies, they were not criminals, they were nothing earthly now—nothing but black shadows of disease and starvation...[They were] scattered in every pose of contorted collapse, as in some picture of massacre or pestilence...I stood horror-struck...
When one man is blamed for the fire in a small shed, he is brutally punished.
The shed was already a heap of embers glowing fiercely. A n***r was being beaten near by. They said he had caused the fire in some way...he was screeching most horribly.
Marlow's descriptions of how the natives are treated share only his horror and pain in watching "men" (for he sees them as men and nothing less) brutalized so casually by the Europeans.
It is for the Company's agents that Marlow seems to experience a sense of condescension. When the brickmaker begins to dig for information, Marlow is confused—then amazed...the man cares only for his own position with the Company, and how Kurtz and now Marlow (he thinks) have threatened that.
Marlow watches the pilgrims (agents) fire with wild abandon into the jungle simply to kill unseen natives. And another point, Marlow blows the steamboat whistle to frighten the "savages" away from danger:
I pulled the string of the whistle, and I did this because I saw the pilgrims on the deck getting out their rifles with an air of anticipating a jolly lark. At the sudden screech there was a movement of abject terror through that wedged mass of bodies. 'Don't! don't frighten them away,' cried someone on deck disconsolately. I pulled the string time after time.
Marlow suffers to see how the natives are treated. His disgust is directed at the Company men and their disregard for human life.