I enjoyed the Pratchett dialogue, but I am not sure I learned from it -- I wind up empathizing with both characters. Are you agreeing with me? Disagreeing with me? What is the converse of a dialogue? I'm confused.

I think part of what bothers me about your Cassini quote is that the claims in the first paragraph are overstated, especially coming from a character who is (presumably) a metaethical nihilist/egoist.

Life is the scum of matter, and people are the scum of life.

Why, is it so wrong to eat things? Eating seems normal and natural to me; an activity to be celebrated. "Scum" is a kind of life that prevents our usual foods from being healthy for us -- it is thus an odd insult for a carnivore.

Nothing matters, except what matters to you. Might makes right, and power makes freedom.

Why? If I firmly estimate that other minds exist, does the existence of those minds depend upon my estimation? If other minds exist, why should what matters to them be irrelevant? What does it even mean to say that "might makes right" except that I plan to ignore the concept of "right"? When, in the course of human events, has the power to ignore morality left people truly free?

You are free to do whatever is in your power, and if you want to survive and thrive you had better do whatever is in your interests.

Really? All the time? Is the world so grim that I must spend all my time eating or face extinction? Surely species and individuals with a significant advantage can spend some of the resulting surplus on frivolous pursuits; what evidence is there that the fate of the world hangs by a razor-thin thread?