New angle:
>The souls of the dead have power
>Necromancers bargain for that power
>What they trade in return is the service of returning a dead soul to life
>Necromancers don't control their undead - au contraire, they're contractually obligated to set them free, sometimes after the dead performs an agreed-upon favor

Necromancy is awful because the souls who are desperate to return to life are usually the last people you want to live twice, and setting their spirits loose on the world is unimaginably reckless.