"I believe the word was, at that time, used synonymously with the word slave."
Not in Conrad's time - slavery had been abolished within the British Empire about 100 years before Conrad was writing, and in the USA about 50 years before.
Conrad did write another book, titled "The Nigger of the Narcissus," in which, if I recall correctly, the term has no perjorative connotations. I think the word is simply what sailors, at the time, called black people. The more refined "negro" might have been used by middle and educated classes.
Until political correctness, the word was generally a denigrating term for black people, but not always. So, in my childhood, (50 years ago,) we had a nursery rhyme "Ten little nigger boys" which had no derogatory significance whatever. On the other hand, if I'd called a coloured person a nigger (not that I ever saw any, except once) my mother would have been cross. Not as cross, however, as she was when I, not knowing the significance, used the word "wog." So, to us, "negro" was the polite term, "nigger" less polite but could be used in context, "wog" abusive and totally unacceptable.
Of course in other countries and cultures, "nigger" was always used abusively.
In respect to the "synonymous with slave" idea, it is perhaps interesting that "Uncle Remus," a (fictional but generally reckoned to be "authentic" as to speech) ex-slave, uses the term nigger as a contemptuous word for fancily dressed town negros, as opposed to the rural plantation negros.
In "Heart of Darkness," if Conrad has contempt for anyone it is the Belgians. They are not, I think, mentioned by name, but there are enough references in the book to ensure that anyone of Conrad's time would have known who he was talking about. The natives are victims of an empire that is run without the "idea."


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