An exert from the revamped and enlarged cosmology I am working on in the background.
What is good and evil all about anyway
That good and evil exist is an undeniable fact, that they are opposed is also undeniable, they attract like and repel the other. What this actually means is debatable at best. Philosophers, scholars, sages and scientists have long investigated the nature of good and evil. For every verified fact about the nature of good and evil there are a thousand hypothesis, base assumptions, ”self-evident” truths and outright fallacies. All that can be said with and degree of certainty is that good tends to deal with things as they might be and evil tends to deal with matters as they are.
Scattered though out the astral sea are fonts of both good and evil. These appear like liquid spheres of a sickly sweet radiant honey or a shimmering dark mercurial substance. These fonts are not an unlimited resource, eventually they run dry. New fonts are found to replace old ones and it is theorized that there is a finite amount of pure good or evil in the universe at any one time. These substances (ambrosia and ichor magna) are highly prized by divine, celestial and mortal beings alike. They are used in a huge variety of application; in spells, rituals, enchanting, genesis, potions and poisons, mutagens and reproduction.
Good an evil exist and may or may not actually equate to wickedness and righteousness; no one knows for certain. Morality in of itself is for most practical purposes subjective but the gods are having non of that; our way is the only way (never mind that "our way" is often inconstant, contradictory, inflexible and morally questionable).
Nercotheists were people who saw the contradictions, questioned the moral values of the divine mandate, doubted their version of the truth and were dubious of the intent of the gods. Finding no satisfactory answers (and the gods move in mysterious is not a satisfactory answer) they rejected the divine covenants as invalid and the gods as suspect.
Now they use both good and evil purely as a resource, they decide for themselves what is or is not moral on its own merits without consulting scripture. In theory it was supposed to be spiritual anarchism but as with all things the burgeoning new state has replaced the gods.
And that is a very good question.Also, if I were to abhor the gods because they want to punish/reward me in an afterlife based on my deeds, instead seeking to take control of my own "afterlife" through necromancy so that they have no power over me anymore and thus cannot "bully" me into being a nice person, then wouldn't I feel similarly about worldly authority who seeks to punish me for my deeds? What makes the lich judge fit to decide over my fate based on my actions, but not the divine judge? Or to phrase it differently: Why are the necrotheists willing to conform to the rules of the undead state instead of their own if they aren't even willing to conform to the objectively given rules of the cosmos?
Part of it is because:
Legalism: That liche is answerable to an elected (and mostly alive) body and has to operate by known rules and procedures, not fiat.
Aspiration: Because those undead are us and we might become them, particularly over the course of multiple incarnations.
Reverence: People don't stop needing spiritual satisfaction just because the gods are forbidden. Some of that awe has been transferred to the undead; thus is very plausible in a society with a preference for ancestor reverence and/or physical gods.
Compulsion: It is a contradiction, a source of underling tension. Social, economic, legal and physical pressures may be employed to suppress opposition and it may not always work. The empire was brought down because of internal strife and this may well have been one of the major causative factors involved.