Originally Posted by Qwertystop
Problem with your explanation: All of the Inflict X Wounds spells channel the same negative energy used in Animate Undead.
Second problem: If I remember correctly, the Plane of Negative Energy doesn't have an alignment trait, unlike the aligned afterlife planes (aka the Outer Planes).
I can second this, not to mention spells like Harm or Enervation and Greater Enervation.
Third problem: Several undead, even in Core, are only Mostly Evil (like Mummies and Mummy Lords) or don't have a specific alignment at all, like Ghosts.
My preferred explanation is that Negative Energy is merely an alternate Úlan vital
, life essence, an equal and opposite to positive energy, like matter and antimatter.