In Joseph Conrad's Heart of Darkness, Marlow's character sees the natives as a race of people who are being exploited by the white race.
When King Leopold II of Belgium established a colony in the African Congo, he proclaimed it was for humanitarian reasons—he wanted to enlighten the natives. While there was mention of bringing Christianity to them—and he even allowed Protestant missionaries to travel and live in the Congo—Leopold was motivated only by sheer greed.
When Marlow travels into the Congo, the Company has hired him to retrieve Kurtz, one of their best agents in securing ivory—in fact, there is some jealousy expressed toward Kurtz by others because no one can compete with the amount of ivory he exports from the Inner Station.
For Marlow, this begins simply as an assignment. He has never traveled to this part of the world before. Though there is some foreshadowing by the captain that takes Marlow to the Lower Station, nothing could prepare him for the way the white men are treating the indigenous people of what was then called the Congo.
Revulsion grows within him over the white man's dehumanizing colonization of the Congo.
Marlow hears the natives described by the whites as "enemies" and "criminals," but he sees men who have been enslaved. They walk like they are zombies—even though their bodies live, their spirits are dead. Marlow is horrified. The men are "...connected together with a chain..." Their eyes "...stare stonily..." They walk past Marlow "with that complete, deathlike indifference..."
Marlow leaves the sight of the chain gang behind, trying to put them out of his mind. We can infer in the reading that he is struggling to maintain a semblance of self-control.
You know I am not particularly tender; I've had to strike and to fend off. I've had to resist and to attack sometimes—that's only one way of resisting—without counting the exact cost, according to the demands of such sort of life as I had blundered into. I've seen the devil of violence, and the devil of greed, and the devil of hot desire; but, by all the stars! these were strong, lusty, red-eyed devils, that swayed and drove men—men, I tell you.
This passage reveals that Marlow is struggling to maintain his composure in circumstances that make no sense. When he sees men...
...scattered in every pose of contorted collapse, as in some picture of massacre or pestilence...I stood horror-struck...
Marlow is not an innocent, untried youth. He has had to face hard times in his life, fighting "devils" of many kinds. This experience, however, has taken its toll.
Marlow cannot look at the natives as the violent Europeans do; he cannot abide how atrociously they are treated. Marlow does not see them as criminals, or lesser creatures. He recognizes not only that they are men, but that those who are driving them—enslaving them—are demons: "red-eyed devils."
Marlow's job is to bring Kurtz back. Had he needed the help of the natives, there is nothing to indicate that he would have treated them badly. Because Marlow is a man of some moral standing (demonstrated by his reaction), the treatment of the natives affects him deeply. This foreshadows the horror that still awaits him on the remainder of his journey. Unlike the Company men, the natives are not a means to Marlow's success or material wealth. They are flesh and blood as he is. And he is mortified not just by seeing what is happening to them, but in realizing there is little he can do about it.