I think what happened with the Iato duke that you play is that he was too involved with Enweil. That is not wrong but I get the feeling that the Light was looking at a more impersonal view of the conflict. They're not exactly the most benevolent in terms of caring for individual human concerns, which is normal since they're not human. They are the Light. Their purpose was...pretty much to defeat the Darkness that they more or less saw as represented in that particular conflict by the three NPC invaders.
So keeping that in mind, they most likely didn't care one bit about prior history, or human relations, only that at the moment, and for that particular conflict, they showed their willingness and determination to fight against the Darkness.
In the case of Hetland, I believe it had to do with them thickheadedly bashing against the daimons and dissolving all ties to the monsters once they got the Light on their side. For Riombara, I have no idea. For Sint, I believe that there was at least one priest who got adamant and managed to convince the youngest and most naive among the Light, Virgil, that they would in fact stand up against the daimons. Which they did. Kinda.
As for Thalmarkin, well, recall that by that time it had more or less been overrun by Undead. Given the Undead GM's parting gloating, it is not too absurd to believe that by then Thalmarkin had already been "marked" to become a future cursed land where the undead would spawn.
Anyway, from the Enweil thing, it was mostly an issue with the Blood Cult I believe. Marta pretty much called the Chenier who spoke to her evil because of the whole unwilling sacrifice thing.
In the end, I think that, like many other things, humans can sometimes be very strange creatures in times of great calamity.