A good novel is not the same as a good essay or a political speech. In an argumentative essay or speech, a writer is expected to argue clearly for or against some sort of thesis. Although some novels are polemical in intent, most good novels are not essays in disguise but explorations of human situations through the sensibilities of imaginary characters. Thus Heart of Darkness is not a polemic "for" or "against" imperialism but rather an exploration of how the characters of both Europeans and Africans were transformed by imperialism.
In the novel, Marlow is appalled initially by the ways black slaves are treated by the Europeans and most readers will share his reactions. He recounts, concerning the "tax" system:
The system lent itself to all kinds of tyranny, brutality and subsequent reprisals by the natives. In one concession alone one hundred and forty-two Africans were killed. The spirit of bitterness and hatred generated in the people was quite terrifying, ...
In this way the novel presents a critique of some aspects of imperialism
The figure of Kurtz, who is central to the novel, is monstrous, but represents not pure hegemony of Europe over Africa but the transformation of Europe by its encounter with or "unconcealing" (in the Heideggerian sense) of the "heart of darkness". The Africans themselves though, are often portrayed as brutal and violent, and so the condemnation of the behavior of the Europeans is not one-sided.
The Intended, though, shows another and positive aspect of imperialism, from the point of view of an African who sees in imperial culture the promise of greatness for Africa in joining European civilization. Although Kurtz himself did not match that ideal, the Intended does provide a positive vision of imperialism.