#because i can't root anything in canon the way i could for the grimleal

[iturbide]

Given you gave sympathetic qualities to Grimleal, I'm sure you could write sympathetic qualities to the Agarthans?

As much as I would love to say yes, the unfortunate answer is that I can’t, because the game gives me absolutely nothing to work with.

The Plegians and Grimleal have a number of things going for them when it comes to expanding their traditions, culture, and religion into something that’s not just “evil villain cardboard standee”:

There are multiple canon sources stating the Plegia is a theocracy established around the Fell Dragon, and further stating that worship of Grima is the faith of the land; while the game only calls the worst of them Grimleal, I think it makes sense enough to define a break between the Grimleal faith (aka the general worship of Grima) and the Grimleal cult (aka the people under Validar trying to resurrect Grima), whose actions do not reflect the larger faith. 

Awakening itself shows us that the Plegians are not evil, first through Mustafa -- a man of honor who cares deeply for the soldiers he leads and who is only fighting for Gangrel because of threats made against his family -- and next through the forces that desert Gangrel in the final battle following Emmeryn’s martyrdom.  While they’re brief glimpses in the overall scheme of things, they speak loudly of the potential that the Plegian people -- who, again, worship Grima as the major religion -- are opposed to Gangrel rather than supportive of his cause.

Though the game doesn’t highlight it, I feel it’s important to note that the villages in the chapter where we meet the Grimleal cultists who are hunting Nowi are almost certainly Plegian owing to where the Shepherds are; the fact that no one attacks them but instead thank them for the warning implies that Plegians are on average decent people.

Heroes strongly implies through M!Grima’s level 40 dialogue that Grima is not an evil entity who’s always been bad, but rather a jaded cynic whose abuse at human hands led to their desire to wipe out mankind.  It’s reasonable to think that any religion established around the fell dragon would reflect far more of Grima’s words and actions before that catastrophic break than after, and equally reasonable to imagine that the people who follow it would use the fell dragon’s early interactions with humanity (where they were most likely helpful and protective )as their model for their way of life, rather than their destructive end following centuries of being taken advantage of. 

This is what I started working with when I turned my attention to Plegia and the Grimleal as a whole.  I know that everything I’ve developed for them is personal headcanon only, but it’s rooted in these facts that show Plegia’s people on the whole as very much Not Evil.  Unfortunately, the Agarthans don’t get that treatment in canon:

The Agarthans aren’t a religion.  As best we can tell, they might be a cult group divorced from any religious connotation.  The fact that every Agarthan is implied to be part of said cult makes it hard to expand anything, especially given how little information we’re ultimately given regardless of the route chosen.

We never get to see any redeeming qualities in the Agarthans.  Every single one we meet, whether it’s Thales, Kronya, Solon, or Cornelia, is utterly reprehensible; it’s only made worse by the nameless group of mages, researchers, and scientists that orchestrated the experiments that ruined Lysithea’s and Edelgard’s lives.  If we don’t see anything that casts doubt on the picture shown, it’s really hard to come up with something humanizing that makes logical sense (especially since I really don’t think there can be redemption after acts that criminal).

All we ever hear about the Agarthans, whether it’s from the Nabateans or from their own mouths, is basically clear-cut evil.  Even in Heroes, Kronya’s dialogues are unnerving if not outright horrific. There is absolutely no evidence provided in any of the canon sources that implies the Agarthans are in any way good, sympathetic, or compassionate even to their own (given how Solon and Thales had planned to sacrifice Kronya for the Rite of Zaharas without batting an eye), which leaves no basis for providing them with any kind of humane qualities, let alone any notion of redemption.

This topic actually came up in the past week or so and I expressed my frustration at the way the Agarthans were presented that made it impossible to extrapolate anything even remotely sympathetic about them.  The above just provides a better outline of why it’s so difficult to establish something like that for them: because canon doesn’t give us anything to work with.