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While the other five Ruinstorm factions are obvious about what daemonic faction they represent, Mirror of Hatred is a bit odder, seeming to represent absolute Chaos without any ideology. I've read in at least two places that it's supposed to be a stand-in for the renegade god Malice (Malal as previously known before copyright issues). Is there any lore about what this faction is supposed to be about/represent?

It's Malal/Malice but not actually because GW doesn't own the rights.
The little blurb says that it represents the insanity of Chaos, hating everything so much that eventually that hatred returns to itself.

Such is the hatred that swirls within the Warp that it encompasses all things, and like the dragon of eternity that feasts upon its own tail, this hatred extends even to itself. To expect rational and sane logic from creatures such as these would be foolish, for Chaos was both its name and nature. Yet, in its self-destructive hatred there was no ally to be found , only a new and more unpredictable foe.

But yeah it's absolutely Malal/Malice.