Is Heart of Darkness a Text which Criticises or Endorses Imperialism?

Is Heart of Darkness a text which criticises or endorses imperialism?
A fair answer to this question requires a review of imperialism and the associated notions of prejudice and racism. Completeness also depends upon understanding the setting of Heart of Darkness: the region now called the Democratic Republic of the Congo in the period around 1900. A version of Conrad's work first appeared in the Blackwood's Magazine in 1899, but was published as a novella in 1902.
Imperialism is the political theory of the acquisition and maintenance of large, multi-ethnic states held together by coercion. This model is typified by the ancient empires of Rome and Greece. Subject states were conquered and integrated de facto into the empire. For example, Rome itself effectively became larger with the incorporation of a new state. In the latter half of the 19th century, European countries undertook imperialism in Africa and Asia under a different model, that of colonisation. …