"They were dying slowly-it was clear. They were not enemies, they were not criminals, they were nothing earthly now, nothing but black shadows of disease and starvation, lying confused in the greenish gloom". page 14 para. 3, line 1. The quote is coming from Marlow, upon arriving at the outer station, and first witnessing the devastation the Belgians have caused the native peoples. He is speaking about the black men, who have been enslaved, dying all around him. He can see the work they are being made to do, and finds it a great horror, similar, perhaps, to what hell...
then they will be out the door, and on the bus to work, and never think about the desolate again, until tomorrow's paper.

These people can not feel the pain of those that suffer. Their hearts have not turned to stone by choice, but molded by the dye a society value has cast the unfortunate "sub- races" into. Not until, the Europeans of the 19th century have walked in the African's shoes, or they start to open their third eye will they every have been able to see that the law's of nature take precedence over the economic laws.