I believe you're misusing or misunderstanding the term necrotic. Necrotic is an adjective applied to things pertaining to the dead. And, as we all should know, nature can pertain to the dead. The dead, as you've pointed out yourself, can be raised into unlife by nature magic and that nature magic can technically be defined as necrotic (pertaining to the dead) because it pertains to the dead. Do you get where I'm going with this or what? Because if you don't, take plague for example. The plague school is a combination of shadow and nature. Nature, or "nature energy", is a part of the plague. It is necrotic when it pertains to the dead and it can (or does) cause people to rot.
Without Shadow (or Void), there wouldn't be any nature or life, and so the Void would be ultimately responsible for death. Shadow can't "always" be necrotic because it didn't (or doesn't) always relate to the dead, but it can be defined as necrotic when it does. Hence, all necrotic magic is shadow magic. Not all shadow magic is necrotic.
What annoys me though is that fel magic is technically an entropic, shadow-related force (or shadow-touched force) and the fel essence of entropic horrors is poisonous. Fel energy would poison the living with a fel sickness or fel contagion that drains life. However, fel isn't treated as an entropic force that brings things to a state of entropic decay and oblivion (a.k.a. death). Fel magic is treated as chaos and chaos, when regarding to in-game, is a multi-school that combines all the forces. It would technically be necrotic. Does necrotic magic not ultimately exist because of Light and Shadow? If anything, necrotic magic is an entropic mixture of the forces of Light, Shadow, Life, and Death. It's basically chaos magic but for living humans instead of living titans. Blizzard's employees are very tricky indeed. They established that the titans of the Pantheon were brought to a state of oblivion by Sargeras' fel storm but fel storm is really a misnomer because fel storms are just fel energies given the appearance of those elements. If fel fire is really an element, warlocks would be using decay to subjugate and weaponize it.