Originally Posted by
hamishspence
And Tyr's "undead paladin" Miltiades had a purpose - fulfil the task he couldn't fulfil in life. After he fulfils it, Tyr lets him pass on to the afterlife - at least till Tyr needs him again.
The way archliches respond to the negative energy of a Rebuke attempt - it can Turn them, or even Destroy them if the rebuker is high enough level - is identical to the way Deathless respond to that Rebuke attempt.
Hence the theory that they're only undead because they're early 3.0 - had they been properly updated into the context of 3.5's BOED, they'd be Deathless.
If you do go by RAW though - you need to be able to explain why they're so vulnerable to negative energy, at least in that particular form. Possibly they're suffused with holy energy, and a Rebuke Undead attempt, being a mix of unholy energy and negative energy, disrupts it.
"One of the most heinous crimes that can be committed" is somewhat in alignment-change territory. Just like destroying souls, which is described as something that only evil people are ever willing to do.
Hence people complaining that a Good undead must destroy themselves or change alignment to Neutral or Evil, if Monte Cook's words are taken literally.