Kurtz is a symbol of the evils of European imperialism.
Hired by the Company to collect ivory, Kurtz collects great amounts of it by using nakedly brutal methods and encouraging the native Africans to worship him as a god.
Kurtz starts by believing the propaganda spread throughout Europe that the Europeans mean to benevolently civilize, Christianize, and humanize the "savages" in Africa. Marlow says Kurtz wrote a long and eloquent report on the subject, which included the words
By the simple exercise of our will we can exert a power for good practically unbounded.
However, as the novella shows, absolute power corrupts absolutely, and Kurtz's exercise of power in the Congo turns him into a monster. He becomes the face of what European imperialism really is when the veil is ripped away. Marlow makes this explicit when he writes,
All Europe contributed to the making of Kurtz.
"All of Europe" includes the pious, hypocritical cant of European propaganda about the noble purposes of imperialism that led Kurtz to Africa under the delusion he could do good. It also means the immense power of the superior technology Europeans had and the Company's greed and drive for profit that corrupted him wholly. This—embodied in Kurtz—is what imperialism really is, Marlow is saying: the wholesale destruction of Africa for financial gain under the guise of altruism.
After a time, Kurtz scrawls the following words over his report:
Exterminate all the brutes!
That becomes his new truth and expresses the reality of what many Europeans really believed underneath the pieties.
Marlow has mixed feelings about Kurtz. He understands him as symbolizing the "heart of darkness" of the corrupt and depraved European presence in Africa. But he also admires Kurtz for the "crystal" purity of his honest comprehension of the "horror" of what is going on. Regarding Kurtz's last words, Marlow says that they
had the appalling face of a glimpsed truth.