World of Warcraft: Chronicle shed a lot of light on the motivations and creation of the Burning Legion. Sargeras had a very clear reason for wanting to destroy everything in the universe, and it ties into the origins of the Warcraft's other Big Bads, the Old Gods.
Moving forward, there will be spoilers for Chronicle, but at this point, we kind of have to just accept the information here as part of the background of the game. If you haven't read Chronicle yet, get on it, because we can't hide this all behind spoiler tags anymore.
Sargeras was hunting demons, as he was wont to do, and found that a group of Nathrezim had begun to serve some other dark force unrelated to the Fel corruption of the Twisting Nether. The Dreadlords had started working with the Old Gods - a group of Old Gods who we wouldn't be familiar with, because Sargeras' reaction was to destroy the planet they had infested, even killing the nascent World-Soul that would have, at some point, become a Titan.
Sargeras wants to see the entire universe burned away because he thinks annihilation would be better than corruption by the Void. He's interested in Azeroth in particular because of the World Soul within our planet - one that is reputedly more powerful than any Titan who ever lived (in fact, the reason our Elemental Lords are such jerks is that this World Soul has been siphoning away the harmonizing "Spirit" fifth element to nourish itself.)
Ok, so Sargeras is taking a scorched-earth approach to containing the Void. Not really a good plan, but one that has some logic behind it. Kind of like some crazy guy during the Cold War who would rather see the world reduced to nuclear ashes than allow Communism to take over.
But the Legion uses the Void.
Voidwalkers have been part of the demonic wheelhouse since vanilla - they're the second demon you learn to summon as a Warlock. But we learned recently that Voidwalkers are not, technically, demons. They're void creatures, more related to the Old Gods than your Imp.
Now, to be fair, Warlocks are all about getting powerful things that are also dangerous, so it wouldn't be too much of a stretch for them to seek out something that isn't a demon as long as its dangerous and useful.
But it doesn't stop at player characters. We've seen the Legion very explicitly merging Void and Fel magic, and recently, so it's clearly not some kind of retcon. Xhul'horac in Hellfire Citadel is basically a Fel-infused Void Lord.
Given that the Legion's mission is all about destroying the universe to prevent the Void from claiming it, how the Fel did they start recruiting void creatures into their ranks?
Consider this: Sargeras has been out of commission for a few decades.
Sure, for a being as ancient as Sargeras, that's not a lot of time, but who has been in charge? In Legion quests, three demons are described as leading the invasion - Kil'jaeden, Balnazzar, and Gul'dan But note that one of those is Balnazzar, a dreadlord/nathrezim.
I wonder if Sargeras didn't really give good enough instructions, or perhaps some of his lieutenants are not holding to his vision quite so strongly. It is possible, of course, that over the eons killing everything, Sargeras lost track of his original purpose and started using the Void to do his deeds.
But I also wonder if there's some intra-Legion intrigue going on. How crazy would it be if the end of the expansion has us restoring Sargeras to leadership within the Burning Legion, rather than fighting him ourselves?
Anyway, something to keep an eye on as the expansion moves forward.