Justified in universe, possibly. Good? No, they're fucking evil on a good day. But that's the point. Sure they're evil, but are they the most evil? No. The entire 40K universe runs on Gray and black morality; there's no such thing as good there.
Oh yeah, and I'm still kinda sad about the Tau becoming mind controlling assholes and the chaos gods shifting from neutral forces of the universe toward evil deities