So, we know Malthael is now the "Archangel of Death", and we know he sees death as the one and only end possible for the mortal kind. But how does he actually control death? Was it always part of its power, or did he acquire it somewhere while exploring the world? I couldn't find a clear answer, and I'm really curious about it.
It's never really addressed. Lore transcripts you find in Pandemonium show that Malthael felt and heard the voices of human souls, which summoned him from Pandemonium Fortress. Whether these were stray souls, or spirits trapped in the Soul Prison, is never stated.
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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
Malthael became obsessed with human souls and humanity's ability to permanently die.
If you ask Tyrael in Act IV about Malthael's Fate, he tells you Malthael was tortured by being unable to find the Worldstone after Inarius stole it, and that "Some say he haunts the silent halls of Pandemonium, forever seeking answers to the unknowable mysteries of life and death."
Malthael's writings you can find in Act V tell us that souls called to him like a siren's song, he checked all over creation and eventually found they were calling him from Pandemonium, where the Worldstone was. He studied this swirling, writhing souls and learned the truth of mortals - no matter what they do, they die, and that is wisdom. His studies of the souls show to him that man can stand for good like any angel, or commit evil worthy of the lowest demon. He does not like that aspect one bit. To an immortal being like himself, the thought of these mortals that are here for but an instant before flaring and dying being able to choose to be good or evil is wrong.
Malthael says that the humans can't be trusted. They are part demon, and demons pervert everything they touch. The humans are therefore corrupt, and not worthy of being able to choose between good or evil. The angels and demons do not choose, and that is how it should be.
With the Prime Evil trapped in the Black Soulstone, Malthael realized that it was the perfect time to end the Eternal Conflict. The demons are leaderless and will be easy prey, but the humans must die, lest they grow too strong.