https://www.youtube.com/watch?v=FHWwfscCpx4
Don’t expect great writing from the crew who made this gem of shocking character revelations. Zo’Vald, the Jailer, the creator and conductor of everything bad ever… is bad . the one controlling the Lich King is like the Lich King! 
How could Sylvanas ever forseen this?!
Yes, the writers forget everything that has been happening - always. It will never change. Shadowlands uprooted what we kinda used as a foundation of ‘that we know’, it turned it all into “you think you knew, but really - the Jailooor did it!” meme.
So there is no surprise that the writers just ignore what demons are supposed to be. Because - everything is at worst, morally Grey…
** TLDR of this part: Headcannon reasoning to why Man’ari works as good “demons”:
What stings, is that if you gave 10 fanboys a week, they would together for a somewhat acceptable coherent reasoning for the whole event. Something like, “Man’ari Eredar, was forged by Sargeras arcane power to remain in control of their current self - yet still to become immortal demonic beings”. Kind of how DHs work. That will be the the pin that makes this plausible mechanicaly in our minds.
Why would Sargeras do this? Demons are inherently egoistical being - they will betray you for their own gains at a flip of a coin. The only thing holding them in line, is the personal power of Sargeras himself. Using these Man’ari beings as his lieutenants and investing the effort to create them, was necessary to be able to lead the Burning Legion. He needed loyal subjects, not demons. Heck, you could even bind it towards SL lore, Man’ari Eredar could be crafted, because Sargeras had this inkling feeling the Dreadlords were not what they seemed to be… that he needed his own loyal agents, created and bound by his power directly.
This creation process chimes with why Kill’Jaden could find it in his heart to feel regret for his actions - as his soul, his very self, was not yet pure demon; at the very end he reflected deep within himself and found the spark of good yet uncorrupted. There was empathy in his heart still.
His death being the Fel released that was forced into order by Sargeras arcane constructs undone. (This would visually show, as Kill’Jaden red form cracking, showing the fel unleashed beneath - then explosion)
The reason Man’ari Eredar are ‘special’ now are all thanks to Sargeras’ intervention in the process of creating a demonic being. They are not demons that now are good, they never truely became pure demons - by design!
This is concludes why and how. Now with Sargeras sealed, their Eredar hearts are able to reflect on their actions - they are no longer a soul under the constant pressure of Sargeras’ will. They have been given time to find themselves, no longer does the Arcane runes keep them swayed. Yet now, the fel in their veins that made them instruments of evil, grows stronger - and fast. They are becoming true demons! (tick tock, goes the clock)
Then you can keep the quest as is, the regret - introspection, the belief they have sinned. That they must repent. Not all Man’ari would come to the same conclusion, on this journey of self discovery. Yet, that some do - seems very plausible if not likely. That there will be those who seek to protect what little good is left within them works. Velen could be their key to bring the Light of Naruu within them, to secure this hold of their souls before it would be too late. Thus, they regained the connection to the light.
Also, you would have those Man’ari who chose to embrace the fel. Let their empathy and spark of good to be extinguished forever. Becoming great coming villains! Perhaps - some in their desperation, go 100% in to find Sargeras, to again feel the soothing pressure dampening their hearts; to deny the atrocity of their actions that they fear to face.
*** end of TLDR segment.
Created in 3min, a lot better than:
“Demons good now. Dont think about it.”
This lack of care from blizzard to at least try to work within the illusion of a framework, is soulcrushing.
