Heart of Darkness is a critique of European imperialism. Marlow's aunt helps secure a job for him with in Africa working as a steamboat captain for a trading company in the the Belgian Congo. She interprets the job idealistically, seeing her nephew as helping in the noble European mission of spreading "civilization" and Christianity to native peoples. The aunt sees Marlow as a saintly figure for going to Africa: as
an emissary of light, something like a lower sort of apostle. There had been a lot of such rot let loose in print and talk just about that time, and the excellent woman, living right in the rush of all that humbug, got carried off her feet.
Instead, Marlow finds himself in a horror story of corruption, exploitation, and cruelty in the "heart of darkness" where all that matters is the profit brought to the company from collecting ivory. Kurtz, a figure who frightens those around Marlow, is tolerated by the company because he brings in the most ivory, and they turn a blind eye to rumors of his brutality and very strange behavior. When Marlow does come across him, Kurtz is dying, but he represents the extremes of European imperialism run amok: the skulls mounted around his camp attest to the cruelty and terror with which he runs his business enterprise as a demigod.
Kurtz represents the reality of European imperialism and greed that Conrad is intent on exposing: while Kurtz has in his possession a propaganda pamphlet about bringing civilization to Africa, Kurtz has written on it "Exterminate all the brutes!" These four words express the real intent of European imperialism in Africa and the level of greed and brutality to which Europe has stooped.
While Marlow continues to lie to the women back home about what is going on in Africa, refusing to shatter Kurtz's fiancée's illusions about her dead beloved, Conrad wrote the novel to expose the truth that European imperialism was an evil system.