Well, I definitely don't agree with your reading of HP, but I'm trying not to just get into an argument about how to interpret the text here because the text is so full of holes that you could drive just about any interpret through it.
But I also don't think this fully answers my question. If "muggle harmlessness and benevolent wizard supremacy" are the underlying ethos of the Gryffindors, that still seems like a better and more justifiable point of view than the ethos of the Slytherins. Even if you think there's something to be said for a degree of wizard separatism, the Slytherins absolutely take that to a place of bigotry and prejudice and, indeed, wizard supremacy. Even if you think that muggle-human relationships are fraught, that's still no reason to act like total dicks towards individual muggles or wizards with muggle parents, and try to exclude them from full participation in wizarding society. But that's such a large part of what Slytherins actually do.
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But I also don't think this fully answers my question. If "muggle harmlessness and benevolent wizard supremacy" are the underlying ethos of the Gryffindors, that still seems like a better and more justifiable point of view than the ethos of the Slytherins. Even if you think there's something to be said for a degree of wizard separatism, the Slytherins absolutely take that to a place of bigotry and prejudice and, indeed, wizard supremacy. Even if you think that muggle-human relationships are fraught, that's still no reason to act like total dicks towards individual muggles or wizards with muggle parents, and try to exclude them from full participation in wizarding society. But that's such a large part of what Slytherins actually do.