In my Current World, Necromancy is a positive thing (The legal parts at least) It operates much like Science does today, through Donations.
A necromancer can splinter off a small part of his soul and mind, and use it to animate an empty unprotected corpse. This corpse is usually bought by the necromancer when it is still alive. The person will sell her or his body, and receive enough money each year to eek out a living, in return, they swallow a small talisman that contains a fraction of the Necromancer's soul and mind. When the person dies, and their mind and soul flee the body, the talisman is released, and fill the now vacant body, creating a Zombie (I need a more culturally appropriate name for it) which the Necromancer can locate, direct and control.
These Necromancers use the Zombies for a wide variety of physical Labor, including Agriculture, (Why use rebellious slaves when you can use Loyal zombies that need no food?), Construction, transportation or Warfare.
Some people don't like the thought of their bodies being used as such, and create their own talisman, that will animate their bodies with small fractions of themselves, rendering the body unusable by others.
The downside to this is the decay of the bodies. They're still dead. They'll still rot unless preserved, and once the body is too far gone, the necromancer's splinter fades away.
Other Forms of Necromancy are a bit more, unethical. (Also depraved, sadistic and nightmare inducing.)