Arnold later confesses to Rishe that he hates war, that the day he first had to kill someone was the worst day of his life, and when Rishe admiring calls his swordsmanship "beautiful" he admonishes her that something intended only for killing should not be described that way. We see it for the first time in this chapter when Rishe is talking about her longing to visit the Empire, but he also has some serious self-loathing over his country's militarism and his role as his father's son in furthering it. He even explodes at Rishe once when he feels she's viewing him as too much of a good guy, attempting to physically threaten her and telling her that if she'd seen him on the battlefield she wouldn't trust him so much (it doesn't work because Rishe has seen him on the battlefield so she calls his bluff).
So he is an asshole, but it seems he's acting as one more out of duty to his role as crown prince and warlord of a militaristic empire. It still doesn't explain why he starts a world war, but then that's the central mystery of the story so I guess we'll find out eventually.