What is troubling about Marlow's diction?

I'm thinking that you are referring to Conrad's repeated racially charged words. Conrad uses words like "nigger" and "savage" for a few reasons. This novella, based on Conrad's own trip to the Congo, was written in the late 1800's. Many of the words considered offensive today were not in Conrad's time. The character of Marlow also considers this region an alien world. The whites colonizing the area dominated a very different kind of culture. Indeed, there was absolutely no attempt made to understand the culture that they had invaded. There was a real sense of "otherness" when Marlow speaks of the indigenous people. Marlow refers to the blacks as "shadows" as he attempts to get his bearings in a very foreign land.