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Haus sees everything. Perhaps there simply isn't anyone more worthy than Abaddon. Which isn't surprising, as he's the heir of the arch-traitor, and who generally becomes a traitor? All sorts of sociopaths who don't care who they kill, like World Eaters, or brainless fanatics like Word Bearers, or a sodomitic, for lack of a better word, bunch of losers like the Children, or a sulky, offended brat like Magnus, who violated a direct order and is therefore unreliable. There are only two Chaos Legions that I find appealing: Alpha Legion and Death Guard. But the former is just the same offended brat who doesn't even acknowledge Chaos, and the latter had no choice and doesn't strive for anything, they are indifferent.
There is no one more worthy than Abaddon, the heir of the arch-traitor, because Chaos itself consists of outcasts. That's why all the polymers fail.