In virtually any other setting, the Tau would be the bad guys - a technologically advanced collectivist society that is utterly bent on establishing a cultural hegemony on the galaxy even as they conquer and annex entire species at a time. All under the leadership of a priest caste that commands fanatical loyalty and unquestioned obedience from any of their subordinates with the merest word. They even practice eugenics, forcible re-education and possibly outright mind control.

It's a wonderful measuring stick for the 40k universe that they're among the closest possible thing to 'good' you're likely to find.