Quote Originally Posted by Qwertystop View Post
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.