And we have plenty stating otherwise. In fact, of the quotes you've posted, only one specifically mentions Sadow being killed by Nadd and it's from an in-universe journal. Most of the others simply state he goes into stasis which does not preclude the possibilty of Nadd being trained by Sadow's spirit. After all, the Dark Side Sourcebook says Sadow went into suspended animation but then mentions Nadd "awakening Sadow's dark spirit". It's far from cut and dry.
Registered: Aug 2014
Location: The balance of the Force
I guess we won't ever really have a definitive answer, but the fact remains, even if he's a spirit, the fact that he's already far more powerful before he gets far more powerful himself is pretty indicative.
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Within your furnace heart, you burn in your own flame. This is how it feels to be Anakin Skywalker.
Nadd being a servant to Ragnos from what I can tell was some belief held by KMC way back in 2005, and from reading the comics there's really nothing that indicates he was a servant to him at all. Only implication I got was how Ragnos was the Dark Lord of the Sith when Kun and Ulic became Sith. Of course IIRC Tales of the Jedi before the Golden age of the Sith arcs were written with the intentions that Sadow wasn't a Dark Lord but rather a mere Sith Magician who was exiled by the Dark Lords, but ofc that was retconn'd.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
The one I was referring to before is like Ragnos'. Vague quotes about being the "greatest" of the Ancient Sith Lords, and it's a character opinion anyway.