Malthael was the former leader of the Angiris council but abandoned his post. He was very well respected by Imperius, but it was obvious to even him that Malthael had fallen to corruption; Malthael tasked his agents with destroying Heaven's portal to Pandemonium, and several Angels were caught in the cross fire. Imperius' honor could no longer allow him to stand by and watch, but his compassion for his former friend also wouldn't let him kill Malthael outright, either. Hence why he tasked us.
One thing I did notice about Imperius is that his voice has changed; more effects have been added since D3 vanilla. I don't know if this is just a cosmetic change, or hints at a change in his character.
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Be it through hallowed grounds or lands of sorrow
All in the Forger's wake is left bereft and fallow
Is the residuum worth the cost of destruction and maiming;
Or is the shaping a culling and exercise in taming?
The road's goal is the dark Origin of Being
But be wary through what thickets it winds.
-Excerpt from the Litany of Residuum;
As Translated by He Who Brings Order