Justice is a fan-made global mod.

Well, Asra is the Goddess of Life and Death, who also controls time (there was something like that), and the undead are closer to her than anyone else.
The problem is that the undead are animated by necromancy, dark magic, and not by resurrection, not by Light, which is why they are vulnerable to Light, but not vulnerable to Darkness.
Ideally, it would be logical to make the red tier of the empire immune to unholy words, because they are kind of enhanced by demons (Biara was in power, demons are everywhere, and the crazy Alaric runs away into the sunset), but that would be imbalanced.
The orcs have a similar problem: the mages mixed humans and demons, and as a result, they have resistance to magic, but vulnerability to both words. It would be nice to make 50% resistance to one and 50% to the other, although the opponent will not use both words in one battle.
There are not many demon worshipers among the mages; the famous ones are Menelag and Fenrig, and Arantir killed the rest, so it makes sense to leave them alone.