Malthael had always been the voice that guide and led the Angiris Council (as well as most of angelkind), he was their leader, whose knowledge and insight was well respected by all angels. As Imperius had stated:
*"Malthael would never had been one without an answer"*
Malthael’s wisdom was a tool for himself to wield in order to lead Heaven and battle the Burning Hells, it was what he relied on and it greatly aided him without fail… until the theft of the Worldstone. For the first time, Malthael was without answer, he couldn’t provide knowledge or insight. He couldn’t answer the crucial question of the Worldstone’s location, the sacred stone that Heaven and Hell have fought eons over. It was the beginning of Malthael’s decent into madness. We see a bit of this during the events of the sin war trilogy, when Malthael was asked to vote, and yet Malthael did not take the opportunity to guide his angelic brothers and sister and instead abstained and that his demeanor radiated with such death that his voice gave chills to the humans present.
Wisdom was his strength and nourishment, yet it also served as his undoing. This was also because Malthael was too rigid and shortsighted. He wasn’t adaptable with the way he saw wisdom (just as you said). Rather than trying to see the wisdom in Humanity (as Auriel sees the hope in them), or understanding that his lack of wisdom pertaining to them isn’t necessarily a bad thing (just as Itherael doesn’t see the fact that being unable to see the fate of Humanity and Nephalem isn’t a bad thing); He began viewing humanity’s eradication as a necessary objective, one that took priority over truly defeating the Burning Hells regardless of the consequences. He spent his time constructing instruments that were capable of shattering souls, such as the soulsmasher, to the point that Tyrael wondered how long himself and the rest of angelkind hadn’t noticed Malthael’s madness.
He plotted to have the black soulstone remain in the high heavens long enough for the Angiris Council to vote on humanity’s destruction, despite the fact that the black soulstone was slowly corrupting the Heavens and causing corrupted angels to be born and twisting the angiris council.
Instead of negotiating, Malthael ordered his reapers to attack the High Heavens in an attempt to destroy the portal leading to the Battlefields of Eternity and prevent the Nephalem from following him. This act caused the death of many angels and even made Imperius (someone who greatly respected Malthael and hated humanity) question his brother’s sanity to the point of believing Malthael was sick and suffering and thus needed to be put down.
Upon meeting the Nephalem in battle and seeing their strength, Malthael destroyed the black soulstone in order to absorb the essence of the Prime evil within himself in order to augment his abilities without possibly considering the following:
- in the case of his death (which happened), the Diablo along with the other great evils will be freed and eventually revive.
- Even if he won, Malthael would’ve likely faced great corruption that would’ve damned him for all eternity.
- Destroying the Black Soulstone also meant stopping his ritual of genocide upon Sanctuary. Which meant that to finish the job, he and his reapers would’ve had to contend with Sanctuary’s remaining survivors as well as possibly his angelic brethern.
In the end, Malthael served as an example, that angels don’t have to undergo demonic corruption to fall. And instead their hubris, arrogance, and foolishness can do the job just as well, if not more so. And I imagine we’ll see the consequences unfold further in Diablo 4.
