Necromancers are not actually undead.

Liches are undead. The difference is that a lich is a necromancer who was afraid of death and decided to become immortal.

They simply control hordes of undead. And they adopted from them the desire to kill everyone else. This gives them power.

Lol.
Undead under the control of a necromancer are just tools, without desires or emotions. You can't learn anything from them. And necromancers who want to "kill everyone else" appear for other reasons, usually because of a desire for revenge (over 9000) or because of a peculiar philosophy (some necromancers from Heroes of Might and Magic 5, Kergan from Arcanum, and a couple of others).