First of all, there's a big difference between "the good guys" and "not quite as horrible as all the others". Second, T'au, Aeldari, and the Leagues of Votann exist. They all have their own flaws, there are literally no good guys, but overall they're fine. If you compare the Imperium with the Drukhari or Chaos, yeah sure you can't get much worse. But saying, even in the Warhammer 40k universe, that anyone who's better than that are "the good guys" is absurd.