Being evil does imply that you cant save the world because every single necromancer in existance wants to destroy, deathknights are a class because of WotlK popularity and in the story there are thousands of them so it would be pointless not to make use of a powerful soldier. Necromancy is a pure evil magic only, deathknights barely use it as it is so they barely get a pass, but a necromancer would be all in and there is no ignoring that.
Forsaken chars dont need to eat corpses at all, if you use the racial thats on you as everything is down to choice.
All the necrolords abilities dont have anything to do with necromancy at all as its all the just shadow damage abilities and nothing necromancer about them, only one to remotely have a connection is the shield, but again were in a realm of death where death is not the end anyway.