Strictly some fun speculation on how Xal’atath and Sargeras’ motives could converge and what everyone is actually up to / why we haven’t revealed more in-game yet.
This requires a little speculation into Titan motives + a bit of lovecraft (Azathoth) + domination rune origins etc. Humor me! Theories upon theories, just some fun until we find out in the Last Titan 
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Xal’atath’s home world was not abandoned by the Light, the Light was taken. The Light-aligned world soul of a celestial body (Sargeras) was twisted by Aman’thul, who used his Order magic to force Sargeras to join his pantheon of cosmic forces. Under Aman’thul’s influence, Sargeras’ was forced to abandon a twin world soul.
In the absence of Sargeras’ Light the Void twisted & corrupted the now lone world soul.
Serving as the champion of Aman’thul’s pantheon, Sargeras waged war amongst the demon worlds, defending planets shaped by Order. As powerful as Sargeras was, he was not fully in control of his own actions. Similar to Anduin Wrynn under the control of domination runes, he was torn, forced to commit acts of violence that went against his very being…his pacifist nature born from the Light.
In a moment of freedom, torn by his actions, Sargeras returns to see the world soul he once watched over consumed by the Void at the absence of his Light. Stricken by grief & rage, Sargeras loses himself to Disorder, cleaving the corrupted world in two and breaking Aman’thul’s control. He sets out to destroy the very nature of reality, disgusted by the plans of Order, Void & Light.
A being of this now void-corrupted world, a child, Xal’atath, spends millennia resenting the Light for destroying her home, for leaving her people. After a millennia finds the Dreamer, Azeroth. The first world soul. The world soul with the power to shape/dream new worlds and re-write reality. Xal’atath plans to crush the Light and make sure no other world can ever be abandoned the way hers was. Betrayed by the Light. ‘Survival is all that matters’.
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Some cool extras + thoughts;
This one is kinda cool. In the Xal’atath animation (Supremacy) where she is recruiting Saldahaar, and is seen as a child. Her eyes glow red. Fiery red. Sargeras fiery red. The pendant she wears mirrors the shape of the red star that we now see from Azeroth (Seat of the Panetheon).
This one is a stretch. Lets assume this N’zoth quote is about Aman’thul, and Sargeras is Light aligned. - “The Light has struck a bargain with the enemy of all” - If Sargeras was a Light aligned being maybe Aman’thul, in his pact with the Light, was offered up Sargeras by the Light, as champion of the Pantheon. Betrayed.
A wishlist item for me. One of the most meme’d upon storylines. Anduin in shadowlands and TWW, would be an incredible parallel to a Sargeras story like this. The cinematic in front of Sargeras’ sword, the slow turn to guilt, madness and lack of faith in the light.
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Edit: This opens up a lot of possibilities for The Child of Light & Shadow prophecy, ending the age of demons. (Xal? Arathor?) If I had to guess it’s Arator but he brings an end to the ‘need’ for Sargeras’ crusade, rather than killing him.
Xal’atath’s hatred towards Alleria being based on her own family grief & dependencies on the light.
Illidan mirroring Sargeras’ rage toward Xe’ra, cementing him as Disorder & Fel aligned.
The Arator Immolation cinematic, the focus on flames, scorched light and fire, the betrayal of the Light when you need it the most. The Sargeras parallels would be amazing.
I do believe Sargeras’ sword being made from bone + anguish would be incredibly awesome (pre-cleanse). Maybe he stabbed the world because anguish can extinguish the power of void? Who knows. If that anguish came from the souls of the world he watched over, I’d be sold.
I feel like something like this would explain years of lore.