Techniques of constructive criticism aim to improve the behavior or the behavioral results of a person, while consciously avoiding personal attacks and blaming. This kind of criticism is carefully framed in language acceptable to the target person, often acknowledging that the critics themselves could be wrong. Insulting language and hostile language are avoided, and phrases are used like "I feel..." and "It's my understanding that..." and so on. Constructive critics try to stand in the shoes of the person criticized, and consider what things would look like from their perspective. 
It might be contended, of course, that the attitude to the African in Heart of Darkness is not Conrad's but that of his fictional narrator, Marlow, and that far from endorsing it Conrad might indeed be holding it up to irony and criticism. Certainly Conrad appears to go to considerable pains to set up layers of insulation between himself and the moral universe of his history. He has, for example, a narrator behind a narrator. The primary narrator is Marlow but his account is given to us through the filter of a second, shadowy person. But if Conrad's intention is to draw a cordon sanitaire between himself and the moral and psychological malaise of his narrator his care seems to me totally wasted because he neglects to hint however subtly or tentatively at an alternative frame of reference by which we may judge the actions and opinions of his characters. It would not have been beyond Conrad's power to make that provision if he had thought it necessary. Marlow seems to me to enjoy Conrad's complete confidence -- a feeling reinforced by the close similarities between their two careers.