This post is going to be about specific spoilers for World of Warcraft Chronicles. Given that this book is likely to be the (first volume) of Warcraft's Silmarilion equivalent, it's got some pretty huge stuff. While I love ambiguities in lore, and the potential for speculation, it makes sense that Blizzard would lock a bunch of this stuff down. The big decision, and something that I applaud for its boldness, even if time will tell whether it was truly the "right" thing to do, was whether to share that with the fans. Blizzard has, by releasing this book, said emphatically yes.
One big caveat to this is that it's likely some of this lore was not locked down 22 years ago when the series first began (hell, I don't think Blizzard even had the concept of Draenor/Outland when they made the first game, and that the Orcs were just pure evil coming from essentially hell.) So the questions raised here might simply be "because they hadn't locked down the lore yet." Some things, though, are recent enough that they open up new avenues to speculation.
Tired of the vagueness? Let's jump beyond the spoiler cut.
First things first: The Nathrezim were involved in Sargeras' fall to despair, but they were not the original evil that inspired this. That evil was the Void. The Void is the opposite of the Light, and is where the power of shadow comes from. Demons are actually not related to the Void. They are beings of the Twisting Nether, which is neither Light nor Dark. However, some Nathrezim had come to follow the will of the Void, within which seem to reside Void Lords - beings that are so alien that they cannot seem to manifest in physical reality - at least not for long.
The Void wishes to convert the physical world into more void, and they intend to do this by corrupting a World-Soul with their Shadow magic. What's a World-Soul, you ask? A world-soul is a nascent Titan.
To do this, the Void Lords created massive organic parasites - the closest they could come to manifesting in a permanent way within the physical world. These parasites you should know more familiarly as the Old Gods. Whatever might arise from that corruption would be inconceivably powerful.
Sargeras found one planet within which a World-Soul was being corrupted by Old Gods. When he did, he destroyed the planet, along with its World-Soul. The Titans were horrified by what he did. Sargeras said that they should do this with any World-Soul they found. He felt that by eradicating all life in the universe, he would protect it from corruption by the Void.
The Titans would not help him in this task, and so instead he shattered Mardum, freeing the demons whose souls he had trapped there, and thus created his Burning Legion.
When the Titans attempted to reason with him, he first killed Aggramar, and then prevailed in his war agains the rest of the Pantheon by using the demonic Fel magic to overwhelm the Titans, killing all of them. The Titans were only able to send one last message to the Keepers on Azeroth - their intention of implanting their souls within the Keepers did not work, and so the Titans are truly gone, except for the potential future one named Azeroth, and of course, the Dark Titan Sargeras.
What's interesting is that this makes Sargeras a well-intentioned extremist, rather than a pure "for the evils" villain. (The Void and Old Gods, on the other hand, do fit that bill.) Yet if Sargeras' plan is to euthanize the universe, why, then is the Burning Legion so interested in corruption?
Whereas the mechanisms left in place by the Titans, like Algalon, have a kind of stony, callous heartlessness, there's no real ill will. The Legion is freaking vindictive. They like to inflict pain, and not simply kill things quickly and relatively painlessly.
Granted, part of their cruelty is a means to an end - while there are some demons who seem to have arisen directly from the Twisting Nether, many demonic races were once mortal ones - the Man'ari Eredar were once physiologically identical to the Draenei, and you can bet that the Orcs would have eventually become proper demons if they had remained affiliated with the Legion.
It is, of course, also possible that in the long tradition of fighting the Burning Crusade, the Legion became crueler. Sargeras may have taken what satisfaction he could out of his dark task by choosing to relish in the destruction and death he was wreaking.
But the other explanation is that demons are just inherently jerks. After all, even before discovering the shadow-worshipping Nathrezim, Sargeras and Aggramar had both been fighting demons for eons. Sargeras freed the demons not because he had any fondness for them - on the contrary, he freed them because he knew that they were just the kind of nihilistic murderers he would need to get the job done. If you think you need to kill everything in the universe, you want to pick foot soldiers who don't mind the task - and might even enjoy it.
The bigger mystery, though, is the fact that the Legion uses Shadow energy itself. The Legion uses Voidwalkers, who are theoretically the same kind of being as the Void Lords. Warlocks learn to summon Voidwalkers pretty early on (though to be fair, Warlocks might not be strictly practitioners of demonic magic, but more just "any magic they can get their hands on, regardless of how dangerous.") Even as recent as the Hellfire Citadel raid, though, we see that the Legion is using void creatures like Xhul'horac.
Isn't this a contradiction of the Legion's mission statement? Aren't these beings the very ones they're trying to "save" the universe from? (A quote from the Vietnam War comes to mind: "We had to destroy the village to save it.")
I suppose there are some potential explanations.
The first is that Sargeras has totally lost perspective, and that he's just so obsessed with killing everything that he's forgotten why, and hey, those Void creatures are pretty effective at killing things.
The seconds is that the Legion might have found a way to turn the Void creatures to their purposes. It could be that the Void creatures we encounter are actually infused with Fel energy to cut them off from their magical source (Xhul'horac certainly has a fair amount of Fel coursing through his being.) Maybe Sargeras even thinks that he might be able to flood the void with Fel - transforming the Void into something more like the Twisting Nether.
The third is that Sargeras ain't here anymore, man. Sargeras kind of went missing after Medivh was killed. He had been possessing the Last Guardian when Lothar took the guy's head off. This happened super-early in modern Warcraft lore. Perhaps under the acting leadership of the Kil'jaeden/Archimonde duumvirate, the Man'ari have been doing things that Sargeras never would have approved of. Maybe he never even made his original concerns clear to them.
Stepping away from Sargeras for a moment, let's talk about the End Time.
Murozond claims that the "true end time" would be far worse than one in which Deathwing succeeded. It's possible that he's simply on team Void - more happy to see Azeroth become a Void Titan than see the universe purged of life. But doesn't End Time look like a version of Azeroth where the life of the world was burned away?
What was Deathwing really doing? And what was his intention?
The received wisdom on Deathwing is that the Old Gods drove him crazy, and they wanted him to break Azeroth to free them. But other than the Titanic containment on them, they didn't want to leave Azeroth. Their purpose is to corrupt the World Soul in Azeroth. Deathwing's act of destruction looks very similar to the kind of "cauterize the everything" that Sargeras was going for.
Now, it's possible, even probable, that Deathwing's intention was to destroy any Titanic resistance to the Old Gods' corruption - that in the alternate future of End Time, the Old Gods were left to their devices without any pesky mortals or keepers there to stop them.
But all of this almost makes me wonder if Deathwing actually broke away from the Old Gods, and that his madness was not cooperation, but a Sargeras-style "kill everything to keep things from getting worse" solution.
It doesn't really add up, given that the faceless had his back. Was Murozond simply motivated by lies? Or is his "true end time" one in which the Legion succeeded in killing Azeroth? Is it ambiguous whether the Legion's or the Old Gods' endgame is worse?
It's a lot to ponder. And I'll admit that some of these inconsistencies might just be that - inconsistencies. Warcraft is a huge fictional universe, and one created by multiple authors over the course of multiple decades, and on top of that, one that is there to serve an interactive medium where gameplay takes precedence over storytelling. Still, I hope that Blizzard embraces these inconsistencies to create some more ambiguity on which to speculate.