5 ideas
Western imagination uses Africa as a foil: the antithesis of Europe, a place where civilization is mocked by triumphant bestiality. Africa is valued as a backdrop that shows off Europe's spiritual grace by contrast. Its people are not considered on their own terms.
Look at who gets language in a text. Conrad's Africans mostly produce 'a violent babble of uncouth sounds' or grunts. In the rare moments they speak English, the lines serve to condemn them, as in the cannibal's request to eat or the announcement 'Mistah Kurtz—he dead.' Withholding coherent speech from a group is a way of withholding its full personhood from the reader.
Critics defend Conrad by pointing to his layers of narrators, which supposedly separate him from Marlow's views. Achebe argues the novel supplies no alternative frame of reference and no sign of authorial distance from Marlow's attitudes toward Africans. Those attitudes therefore stand as the work's own. Irony that exists only in critics' defenses does not count as irony in the text.
A novel that celebrates the dehumanization of part of humanity cannot be called a great work of art, however refined its style. For Achebe, a work's moral content is a condition of aesthetic judgment. It is not a separate issue that brilliant technique can outweigh.
Racism in a celebrated text can go unremarked for decades when its assumptions match the readers' own. Its invisibility to critics measures how normal those assumptions are. It does not show that the racism is absent. Scholarly consensus about a canonical work can inherit and preserve the prejudices of its original moment.