The original flavor of DnD necromancy was slavery at best, with the understanding that if the necromancer loses control the evil spirits will try to murder everyone they find.

And the DnD cosmology pretty much specifically rejects moral relativism.

The actions are evil because the literal gods of goodness literally say it is evil, and they do actually know more than you.

Of course, your personal settings are free to do whatever they want.

You can have all the genocide you want and go on about “perspective” as your paladin crusades through the holy lands to get the necromancer more corpses for his personal army if that’s what gets you off.