Symbolism, Imagery, Allegory
When Karl Yundt raves about the terrible injustices that some humans commit against others, he uses the image of eating and cooking flesh to describe it. Humans, he claims, are cannibals, constantly participating in capitalism so that the rich can keep "nourishing their greed on the quivering flesh and the warm blood of the people—nothing else" (3.44). This speech really skeeves out Stevie, who can't draw a distinction between symbolic and literal meaning.
Also, there is a hint of cannibalistic imagery in the ravenous hunger Verloc experiences after causing Stevie's death, cutting pieces of roast beef "without restraint or decency, cutting thick slices with the sharp carving knife, and swallowing them without bread" (11.92). Blegh. This guy needs to learn some table manners.
The narrator lingers on the image of the Verloc's carving knife, which Mr. Verloc uses to cut slices off a roast in order to gorge himself. Winnie stabs Verloc with this selfsame carving knife, and Verloc's blood runs off of its handle in a particularly gory and vivid scene. In a sense, the book shows us that Verloc's time of gobbling down roasts has reached the end. It's time for him to become the roast.
Through this imagery, Conrad is actually making many of the same comments that Karl Yundt makes about capitalism. The narrator might not like Yundt, but is happy to use the old man's cannibal motif to talk about how awful the world is. Cannibal imagery basically represents the dog-eat-dog (or man-eats-roast-man-becomes-roast) mentality of modern times.
There's only so much money to go around, so everyone scrambles over each other to get it. But worse yet, the people who do have a lot keep making their money off of the efforts of workers who have nothing. That's kind of like cannibalism: using other people's bodies to fulfill your own greed and insatiable appetite.