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Did Malthael and Itherael get mixed up?
Wouldn't it make a lot more sense for the Angel of Fate to become the Angel of Death, since the two subjects are so closely related? Fate isn't even a virtue like Valor, Hope, Justice or Wisdom, Fate is just something that happens regardless of morality.
There's also the "Pool of Wisdom" and Library of Fate" when it feels like it ought to be the other way around.
Nevalistis
Without delving hugely into it, I'd like to recommend you check out Storm of Light by Nate Kenyon. It covers the events between Diablo III and Reaper of Souls and you get some perspective on what being the Aspect of Wisdom means through Tyrael's eyes (after he takes up the mantle at the end of D3).
It's not wholly unmaddening, and Tyrael spends a fair amount of the story attempting to piece together Malthael's thoughts and motivations behind his disappearance. Probably my personal favorite of the Diablo novels.