What is troubling about Marlow's diction

I think probably you mean his references to indigenous peoples can seem troubling to modern readers. Conrad writes about European conquest of the world with a tone of white superiority, "The conquest of the earth, which mostly means the taking it away from those who have a different complexion or slightly flatter noses..." Still we must take this in context. In 1899, much of the world's "uncivilized" places were yet to be explored. The "norm" was white and decidedly European. Marlow's diction merely reflects the realities of the time.