I ask you this now: what does true good stand for without true evil to, by nature, balance the force and harmony of light and dark?
Malthael was the leader of the Angiris Council (and subsequently, the Heavens themselves) and the Aspect of Wisdom. He was the first angelic being to ever breath the new air and bathe in the first streams of light and time-the first born from the Spine of Anu, or the Crystal Arch, whilst Mephisto was the first to bathe himself in the inferno of the Hells and time-the first spawned from the seven heads of Tathamet, and was the Aspect of Hate. Malthael was said to be a calm and guiding figure, leading the Council to victory with his brother Imperius with a steady hand (though Imperius would rather have led it to victory with rather not plans hatched carefully, but a bloody massacre and war cries). It was known that he was the one to find the Eye of Anu and name it The Worldstone, and he cherished it above all things, perhaps even moreso than his own Wisdom and place as the leader of the Heavens, more than any other being could ever fathom cherishing it. For the most part, he remained silent, and thus Imperius often would take command and shoulder his elder brother out of the way, though Malthael simply alloted this and watched as Imperius gave brash orders and commands in a barking, sharp voice. However, when the man did speak his voice, it was recorded to be quite soft, yet commanding and musical, frothing with the wisdom he held, and all who were around stopped to listen to the wise one. He was lost to the Heavens the moment the Worldstone was destroyed, and soon left for the final time, leaving Imperius as the ruler. His departure fractured the council horribly, and to this day it remains unrepaired, as if it were a wound on an angel, filled with corruption, at the hands of an unskilled healer with an eye not trained to detect this corruption. In time, Malthael brought himself to Sanctuary, and soon became disgusted with the humans, a strong antipathy forming for them (though as the leader of the Heavens he cherished all life), growing like a parasite does in its unwilling-and usually unknowing-host, eating away at him slowly, until the dislike became a hate. He took on the Aspect of Death (it should also be noted that during the Sin War, Malthael was dubbed as the Angel of Death by many in coinship of how many demons he had relentlessly slain on the battlefields, perhaps even so through injuries that were perhaps grave to him), though he did not activly attempt to murder the humans. He did, however, interact with them, ("Rumours persist that these baleful breeches were won from Malthael himself in a game of chance. Though the pants offer a significant advantage in combat, they never cease draining the vitality of those around them, as the unfortunate cardship learned only after bringing about the demise of his entire household. It would appear the Angel of Death is a sore loser." - Abd al-Hazir, on the Death's Bargain breeches from Diablo III) apparently even making small bets with them. As time passed, however, these meddlings ceased, and Urzael, along with Malthael's most loyal followers, were sent by Tyrael to search for Malthael, as the Heavens were, well, crumbling at least somewhat, under Imperius' leadership, and Urzael recorded that his master's presence was "sorely needed in the Heavens" (let it be known that the part about the Heavens crumbling under the guide of Imperius is simply me inferring, but due to the nature of Imperius as a leader, and a person, along with the lack of evidence to determine what Urzael meant by this, and finally, the transition that I am sure would have shocked the Heavens of Malthael's calm and orderly ruling to Imperius' hectic and brazen ruling, I can only guess that Imperius was not fit to be the ruler yet and thus Urzael, being tired of Imperius' regin, sought to return Malthael safely as the holder of the Heavenly throne). As Urzael traversed, he came to Sanctuary first as he knew Malthael of late had gained an obsession with humans, and though he did not find Malthael, he found humans and soon contracted a dislike for them as well. When at last he found Malthael, he was not surprised to find he felt the same of the creatures known as humans, and soon they conspired to destroy them, leading smoothly into Act V of Diablo III: Reaper of Souls, where both were ultimately defeated by the Nephalem.
But there's a few questions we can (and should, to neaten things up a little) ask here.
Such being, why did Malthael attack the Heavens? He did it not to eliminate the Heavens, as he believed he was doing good for the Heavens (more on that later), but instead to abolish any humans there, as news of Diablo's attack on the Heavens and the mortals who'd saved the Heavens clearly had reached him. He figured that the humans would stay in the Heavens a little, if not to at the very least clear out more demons. This being noted, he sent his Reapers to the Heavens to destroy not only any remaining demons, but humans as well. The Reapers, from what I believe, attacked a few angels as well, perhaps they felt bold, and this alerted Imperius. Now, to clear up what I mean by his thought process that he was doing the Heavens good and not evil, I'll have to start from the beginning. Malthael was known widely as the embodiment of the Heavens as a whole, and, like his brother Imperius whom he argued with (and perhaps, similar to Tyrael and Imperius, got a little physical with) over their conflicting views, sought out to protect from any foe. Even upon departure, he still felt this way. Upon observing the human race, he saw they had the choice of good and evil resting within them, and they chose evil en masse. Time skip to Post-Reaper of Souls, Malthael believes that with Diablo's defeat the humans, due to the power they have to potential to unlock for themselves, and the evil deeds of many, are the last evil in creation, and destroying them would leave the Heavens truly victorious in the Eternal Conflict. He cared not about the human families, legacies, for all he saw was the work of evil at its finest, and he wished to destroy this in the name of Anu and all things Holy. I believe Imperius called upon the Nephalem for he had little to no faith in them, and doubted that if he told them to kill Malthael they would suceed to kill the most powerful being in all of creation, who would most probably make Diablo himself, even as Legion, look no more harmful than a newborne kit. I think it was his way of weakly retaliating against his brother's attack on the Heavens, or perhaps a twig to simply feed the fire of suffering.
But what if Malthael wasn't killed, and suceeded in destroying all of Sanctuary and the rest of the Burning Hells?
For a start, one of the producers of the Diablo games has confirmed that you cannot kill Diablo forever, but that aside, I believe that in the end as a result, all balance would ultimately crumble to dust-as true good cannot suffice to exist without coexistance with true evil to balance it. At first, the Heavens would rejoice and call him a hero for destroying all evil, but this good tiding would only last for so long as all good things do. The Heavens would be unstable due to the lack of evil and begin to crumble in upon themselves, forming two sides, and once more starting the Eternal Conflict, as it is named "Eternal" for a reason. With no evil to fight, the Heavens would naturally balance themselves by dividing amongst themselves and raging a war perhaps feircer than that of the demons waged against the angels. I think it would, too, affect the Crystal Arch drastically, whether that be by dampening or amplifying the light so drastically from it that it would have dire consequenses. Many angelkind would die, and perhaps from those ashes would rise new demons, as creations way of balancing the good and evil, though in the case no demons were spawned, it would be Heaven's legions pitting against Heaven's legions in a massacre more bloody than any war waged between angels and demons before. The angels, in turn, would ultimately be their own doom, destroying all living including their own brethren like the demons were so known to do. Thus, though Malthael truly believed he was doing right for the Heavens, he failed to see the drastic outcome of his actions had he suceeded.
Let me know in the comments how you think this theory was, and if you liked this one, be sure to stay tuned for more theories!