Satori said:
These topics are continually debated to no end...but it creates some very interesting RP scenarios.
And always will be, for our pleasure
Part of the problem is that D&D (or rather the gamers) often require certain overall rules about stuff which is just subjective to the authors themselves, and it's always better to leave it as a setting-specific or campaign-specific decision. The books try to come out with new options, and it's gamers' fault if those options become rules you can't escape... That's the case if a DM freaks out "oh my god they released a good lich in book X, I have to re-design my entire world if undead can be good!"
Furthermore, D&D rarely invents something truly new, the ideas are rather gathered from folklore, legends or famous fiction. So different specific undead have very different background:
- some undead are evil because they wanted to cheat death, and that's evil e.g. from certain religious perspective (e.g. the archetypical Lich is someone who didn't want to accept death at any cost); however in a world with common resurrection spells, it may not be so evil in principle, and it could be conceived a good lich who stays in the world to help the cause of good; at that point, good/evil may depend either on (1) what the NPC was since the start or (2) what means are used to obtain lichdom
- other become undead because they are evil (instead of the opposite); perhaps Ghouls are those so depraved (cannibals) which were unaccepted even in the afterlife
- necromancy (re-animating) may be evil because it goes against nature or gods, and often against the will of the dead; it may also expose the dead to further chances of doing evil (esp. if they are not in full control of themselves, e.g. skeleton/zombie) after the soul is supposedly already clean
- ghosts have various background: from "person who did something bad, and needs to make up for the evil done" (evil), to "person who was done something bad, and cannot rest properly" (good)