Adria speculated that Malthael knew that Tyrael was responsible for the Worldstone's destruction, and knew why he'd been forced to carry out the action he did.

Malthael did wander these halls, but in madness, as humanity obscured wisdom from his vision. Some speculated (correctly) that he haunted the Halls of Pandemonium, seeking answers to the unknowable mysteries of life and death. His departure broke the unity of the Angiris Council. Just prior to this, he had apparently taken a greater interest in humanity. Malthael disappeared after the destruction of the Worldstone. Malthael abstained from voting, but the end result was that Sanctuary and its denizens would be allowed to exist, free to choose their own path. In the aftermath of the battle, the Angiris Council convened to discuss Sanctuary's fate and that of its inhabitants. Malthael watched on, and beheld the existence of humanity. Towards the end of the Sin War, Heaven became aware of the existence of Sanctuary, and in the final battle, the forces of Heaven, Hell, and the Edyrem clashed against each other. Deckard Cain would later speculate that the existence of the nephalem clouded Malthael's vision or opened his eyes to a new truth, or that there was a connection between the nephalem and Malthael's chalice that the angel became aware of. Following Sanctuary's creation, while it was originally hidden from the High Heavens, Malthael's mood darkened. The coupling of angel and demon would lead to the rise of the nephalem. Malthael became consumed with discovering where the artifact had disappeared to, and as the Archangel of Wisdom, was tortured by his inability to find it. The result was that under the leadership of the angel Inarius and demon Lilith, a number of rogues stole the Worldstone and created their own realm- Sanctuary. Not all minions of Heaven and Hell were content to be bound by the Great Conflict. Malthael abstains from judging Sanctuary ( src)

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