Since no one else did, I'm going to explain a bit about Dungeons & Dragons metaphysics, at least how they worked in previous editions.
You know how there are some fantasy settings where it's said that just about everything is made of "elements", like Earth, Air, Fire, and Water? D&D has those, and they each come from an Elemental Plane, which is a sort of parallel reality composed entirely of that element. In addition, however, D&D has the Positive Energy and Negative Energy planes. Positive Energy is basically life energy, and it's present in every living thing; negative energy is antithetical to it, but it's what grants life to the undead. A cleric's Cure Wounds spells channel positive energy, which is how they heal the living and harm the undead, and the Turn Undead power also works similarly.
This is why mindless undead like skeletons and zombies are treated as inherently evil. They're powered by negative energy, which pollutes the world around them, and also grants them an instinctive malevolence to the living which could drive them to kill if they aren't controlled. Spells to create undead slaves are considered evil for the same reason.
However, that doesn't mean all spells in the "Necromancy" school are evil. There are a whole lot of them with many different effects which don't include creating, summoning, or controlling the undead, and many of these other spells lack the "Evil" descriptor that Animate Dead and its variants have. Even healing magic was considered a type of necromancy in some editions, though others listed it as "Conjuration (Healing)" while the very similar Inflict Wounds spells remained Necromancy.
Incidentally, the elemental and energy planes are collectively known as the Inner Planes, representing the physical substances that make up D&D's Prime Material Plane. This is in contrast to the Outer Planes, which represent philosophical concepts like character alignments and serve as the setting's various heavens and hells.
