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Mostly because necromancy in its core defies the cycle of life. Humans believe that everything have its beginning and its end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse.

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes. Those people are evil.

Mostly because necromancy in its core defies the cycle of life. Humans believe that everything have its beginning and its end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse.

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes.

Edit to answer edited question: Remember that what is good or evil is made by society.So your examples of Sauron and Lich King are considered evil by their enemies. For their "people" they are good.
What is good? They cap the amount of casualties by recycling warriors, don't push economy into spiral by moving goods to war effort (dead soldiers already have their armour and they don't need to eat).
Also punishing dead enemy warriors by forcing them to fight on your side is a major psychological bust for your people "Don't let our boys die for our country" and down for enemies "We have to fight our brothers!"

Mostly because necromancy in its core defies the cycle of life. Humans believe that everything have it's beginning and it's end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse.

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes. Those people are evil.

Mostly because necromancy in its core defies the cycle of life. Humans believe that everything have its beginning and its end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse.

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes. Those people are evil.

Mostly because necromancy in it's core defies the cycle of life. Humans believe that everything have it's beginning and it's end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse.

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes. Those people are evil.

Mostly because necromancy in its core defies the cycle of life. Humans believe that everything have it's beginning and it's end. By being a lich or raising the dead you show that there is no end. And your death release you from your living duties.
In the same manner the stories of ghost who can't rest until they finish their earthly tasks. But with ghost they act on your own while necromancers and liches still have power over a corpse.

It's like when you wake up and say "5 more minutes". And someone will not let you rest those 5 minutes. Those people are evil.