40k could do a thing about a far-off colony world that nobody knows about, that gets taken over by the imperium.

Maybe one of the "evil" alien races even knows about those humans but leaves them alone because they aren't actually evil, just very violently fighting against their own genocide from the imperium.

Maybe to the extent that those aliens are evil, it's because they had to become that way to survive. And most of the lore ends up being just imperium lies.