Lore: The Shadowlands and the undead

Why do only some of the minions from Castle Nathria are counted as undead?
If Morlocula and Kel’Thuzad are undead, why Draka, Olgra and Pelagos aren’t?
It doesn’t make any sense. Aren’t they all dead?

In the Buu saga, is Goku Undead?

Ghosts and wisps and the wildseeds (basically animal spirits) are undead, as well as the val’kyr, why not other souls? That’s my main gripe.
In other fantasy series, sure, undeath is a unnatural mockery of both life and death. In Warcraft, since the original undead are restless souls that got deformed in Maldraxxus to fill the ranks of the “afterlife army” so in this sense isn’t undeath as natural as death itself?

Perhaps the better question is: in the Buu saga, does anyone actually care if Goku is Undead?

Dead? Yes-ish. Undead? No, not really, at least the ones you mention. Not in the sense of the Scourge (or Forsaken). They are in the land of the dead in their eternal afterlife, not planning on making a quick comeback trip to Azeroth to run around with half a jaw missing.
No spoilers or anything, but Pelagos is DEFINITELY not undead.

Questions like these make me wish we had a dedicated lore/story area in the forums.