T'au are basically Brits screaming Orwell Animal Farm 1984 so hard that they accidentally made the T'au based. Literally every sentence about them though is trying really hard to imply something evil behind it, or somehow implying that Ethereals enforce their philosophy of equality and respect with Jedi mindtricks.

The 9e codex went really hard in on the T'au doing controlled breeding for eugenic purposes, but never explained how it actually worked. Of course, GW being Brits, they've kinda done this for just about every faction at this point so it's hard to argue that it makes the T'au particularly evil in the setting.

If we defeat the capitalists in the marketplace of ideas, then their armies will put down their guns and join us because our vision is so pure and good. /s