First, I don't think we should talk about Shu as though he has agency. Shu is at the mercy of his writers and will say whatever they think is entertaining. Distaste towards Indigenous people is a pattern in Shu's writing. Not because the writers think it's a character flaw to be taken seriously and addressed respectfully with him hopefully growing past that mindset, but because some people at Happy Elements and some of the fandom get a kick out of racism. As a result, Shu has unfortunately become a tool through which offensive jokes are made. In Snow Globe, we hear from Mika that Shu looks down on Indigenous cultures. I don't think he would make that up. Mika's shock at Shu's "homage" to Indigenous people and assertion that Shu thinks poorly of them is a reflection of his observations as someone close with Shu. In Shinsekai, Shu has the order to not insult the locals in Touhoku. There's many Japanese people who look down on Touhoku natives and perceive us as uneducated and unrefined. This comes back to the fact Touhoku has historically been home to Emishi and Ainu people. Do you know where the word "shogun" comes from? It's shorthand for seiitaishougun, which means "commander-in-chief of the expeditionary force against the barbarians." Who were the barbarians? The Indigenous people in North-Eastern Japan, Touhoku included. Japanese people have long looked down this part of the archipelago because of this history, and this attitude holds true for many (but not all) today. Our dialects that share features with the Ainu language are frequently mocked and associated with being "backwards" and "uncivil." Some might even say, barbaric? Shu apparently thinks poorly of people from Touhoku. All of this adds up. Even if you dismiss that he directs a slur at Rinne because he uses it non-exclusively (though do remember this word has a loaded history and is a favored insult by racists, and we know there's some of those at Happy Elements), these other glimpses into his world view exist. This is how Shu is characterized. It's unpleasant but it's part of who he is, just like everything else the writers have done with him.