There are very distinct differences between the Belgian traders in Conrad's Heart of Darkness, and the Imperialists in Achebe's Things Fall Apart.

In Conrad's Heart of Darkness, though Leopold II reasoned that "seizing" the Congo for Belgium would ultimately help the natives, the truth is that nothing was established to help these people. They were exploited as slaves and mercilessly butchered or worked to death, and those in charge felt no remorse. The Belgian traders wanted to harvest the natural resources in the Congo, particularly the ivory trade—in doing so, they became obscenely rich on the backs—and over the bodies—of the natives.

Six black men advanced in a file, toiling up the path. They walked erect and slow...I could see every rib, the joints of their limbs were like knots in a rope; each had an iron collar on his neck, and all were connected together with a chain...but these men could by no stretch of imagination be called enemies. They were called criminals, and the outraged law, like the bursting shells, had come to them, an insoluble mystery from the sea...the eyes stared stonily uphill. They passed me within six inches, without a glance, with that complete, deathlike indifference of unhappy savages.

The repression of the natives is obvious to the narrator (Marlow); the men carrying the earth are chained together, working as slaves. They are starving and there is a look of death in their eyes. Marlow is greatly disturbed by what he sees. He moves farther on looking for a place to rest.

Black shapes crouched, lay, sat between the trees leaning against the trunks, clinging to the earth, half coming out, half effaced within the dim like, in all attitudes of pain, abandonment, and despair...this was the place where some of the helpers had withdrawn to die.

They were dying slowly—it was very clear. They were not enemies, they were not criminals, and they were nothing earthly now—nothing but black shadows of disease and starvation, lying confusedly in the greenish gloom.

There is not one redeeming aspect to how the traders treated the natives. This is a continuing theme in the story. Comparing these images with those from Things Fall Apart, we see distinct differences, with similar results.

In Achebe's novel, Imperialism arrived in the name of Christianity and Colonization. The idea of colonization is that the "civilized and advanced" race brought—with their "protection"—better ways to live, as if the traditions of the Nigerian tribes were deficient, although their tribal rituals and beliefs had served them well for many years. Christianity also had a tremendous effect on tribal life: drawing to question the old ways and pulling away those in the tribe who were already not necessarily successful among their own people—but still members of the tribal family. Christianity shook the time-honored beliefs of the natives, destroyed the tribal way of life and changed the face of Nigeria forever.

The white man is very clever. He came quietly and peaceably with his religion. We were amused at his foolishness and allowed him to stay. Now he has won our brothers, and our clan can no longer act like one. He has put a knife on the things that held us together and we have fallen apart.

Additional Source:

http://www.enotes.com/things-fall-apart/summary