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.
There is such thing as good! It's just locked up in grandfather nurgles closet being used to test the latest and greatest plagues and sicknesses before they're welcomed upon a unprepared galaxy.
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