Yrael
His story is ultimately rather straightforward. He had been a grunt in the celestial legions during the times of the Great Empire of the Dawn, which had been a mortal run empire in Planetos and is the predecessor to Yi-Ti (though covering a much larger area). In these times, most people were Good and Righteous, or at least trying to be so. In essence, the realm was the closest a mortal society could get to achieving these high ideals. However, it didn't last and Yrael is one of those who lived through the Sundering from the mortal perspective. He was around when the realm shattered and people turned on each other, something that deeply marked him.
To him, the Upper Planes had won the great war with the establishment of the GEotD and it's millennia lasting golden age. They had fought for their morals, their principles and all the other things a proper Archon believes in and made them reality. But now he had to conceive of the notion that it didn't last. That it wasn't enough. Many other Archons fell in those days, blaming mortals to have squandered what they had in some fashion and joining Baator, or going so far into despair that they joined even the Abyss or Abaddon. Not Yrael though. He still believed that the fight could be won and while he acknowledged that the GEotD was not as flawless as he used to believe, he still thought it was the right path.
With the spheres out of order and the Upper Planes in shambles, he went into hibernation like nearly all other celestial survivors on Planetos, only reawakening upon the surge of magic returning.
Now the world he saw was pretty much the exact opposite of the good old times. Slavery, demons running wild, depravity, sin, egoism, cruelty for the sake of cruelty... He had left a world in which these things were a reality, but not where people reveled in them. So he set out to fix things as much as he could, putting on his best Archon Boyscout face and trying to improve upon the shitheap he was now ruling. But it wasn't easy. He needed to compromise. Sometimes he needed to do things that were not really just or turn a blind eye to bad things, because he needed to do what was best for everyone, not just what he felt was right.
The dissonance ultimately resolved itself when he saw the broken Heaven with his own eyes, as opposed to the second hand tales of it's defeat. He could explain away the fall of the GEotD by assuming it had flaws. He could even apologize his own questionable choices as ruler of Mantarys since he was forced to work with a world that had few if any lines it wouldn't cross. But Heaven? Heaven used to be the strongest of the Outer Planes, bolstered by the strong Lawful Good GEotD. It didn't need to compromise with anyone and yet it had fallen too.
So what did this mean? Were his ideals not enough? Was being Good and Just not a guarantee of victory as he had always believed? Or was Heaven itself flawed too? Asmodeus certainly believed so and he had given many a passionate speech about these things while he was still one of the leaders of Celestia. He had spoken of compromises that had to be made to ensure victory, of prices that were distasteful but had to be paid to beat back the Abyss. Back then they thought him wrong and they felt assured of that when he and his followers couldn't win the Bloodwar either, despite throwing out most of their ideals and turning to spite and cruelty to punish those who they felt were aiding their enemies.
But Heaven was broken and Baator reigned supreme. So had Asmodeus maybe a point? Had they failed by putting the purity of their principles above the necessities of reality? Viserys certainly seemed to believe so, because he also kept talking about necessities and pragmatism, about prices that were worth paying even if they felt wrong. To him Yrael had listened, because he was willing to entertain the notion that it would be necessary to reach again the heights of the golden age he knew. Because he saw that on the blood of fiends and monsters, above the graves of slavers and tyrants, a better future was being built. It was far from perfect and it was steeped in blood, but it was better then what came before and didn't that mean that said blood had been spilled for a good cause? Didn't the end justify the means?
And this is why he fell. Because he realized that he had followed another Asmodeus and that the leaders of Celestia of old would have cast him out just like they had before. He realized that he was willing to put aside his ideals and morals for the right price and that he no longer minded to do so. To him, the ends did now justify the means. He would take a flawed realm over the madness and chaos he had seen. He would take a law that was uncaring over one that was just another cruel tool of torment. He lacked the spite and bitterness of those that were cast out alongside Asmodeus, thus he fell not to Evil like they did, but he had lost his faith in the Good all the same.
Yrael is in the end a character who has begun, like all Archons, as a Paragon, but was tempered and in a sense lessened by reality. He will still fight for what he believes is right, but he is willing to do distasteful things to reach something that is not necessarily good, but at least good enough.
What Yrael isn't is a poor tormented soul that longs to be Good again, or some kind of hamfisted edge-lord metaphor for Good things being unable to exist in reality. He is a character with his own history, desires and personality, who has made a conscious choice to walk his current path and is happy with it, even if others can't understand it.
Lucan
He is pretty much the polar opposite of Yrael in his path. He began as the de facto enforcer of a Lawful Neutral deity. The Father told him what to do and so Lucan did these things, because what else is a servant of a god supposed to do? He had been given a direct divine mandate and orders. So he went around and preached, solved the issues of the small folk here and there and made sure that all withcraft is either brought to heel or eradicated, just as the Father wanted. For after all, how can there good witchcraft? The Seven clearly are the moral authority and they clearly condemn sorcery.
However, Lucan began to doubt. For every Charm abusing conman or cursing witch, he found a healer or some scholar who just minded his own business. Not all of these people seemed really to be bad people, but neither did they want to join the Seven. Some were willing to give up their powers, other not and the latter category he killed on orders from above. But why? Why did he have to kill a healer, just because he didn't want to become a Septon or give up his ability to help others? Was that not work pleasing to the Seven, the Maiden and the Mother in particular?
So he summoned his first few angels and they were... not quite what he was expecting. They were certainly powerful and righteous, but they had this strangeness about them that just seemed lacking in some way. They were doing things because the Father or Lucan told them to do things, be it directly or from the Seven Pointed Star, but it was as if they only went through the motions. There was no heart in it, just obedience. No real compassion for the people.
It was around this time that he had contact with Danelle's predecessor, seeking answers from his fellow Chosen. She had a quite different perspective on things, the Maiden setting quite other targets and using other tools then the Father, but while that gave him hope, it didn't answer his questions. Her death was quite a blow to him and he was in quite a crisis as a result of it. On the one hand, he was very much doubting his faith, unsure if he was somehow not understanding the work his god had given him properly, or that it maybe was some kind of test or trial for him. As a combination of his personal doubts and the desire to call forth a champion that would help other Chosen too, as his own angels were very much the Fathers and only the Fathers, he decided to call Baelor.
However, Baelor the Blessed was a mess. Neurotic, stubborn, absentminded and barely functional as a being. He slavishly obeyed the Seven Pointed Star, but could offer no insights into it, even chastised Lucan for thinking about it instead of obeying and praying to the Seven for guidance. The man was basically every issue that Lucan had with the angels of the Father, just so much worse. When Baelor then got it into his head to pray for more power and divine guidance to vanquish his own family for the crime of apostasy, he was pretty much giving up on him. At least Baelor seemed uninclined to do anything at all, instead doing day long prayer sessions before so much as leaving the room, so he assumed that as long as nobody would actually disturb the nutcase, he would stay put for the time being.
Instead he made his way to Heaven, which also for him facilitated his change of alignment, but in the other direction. Suddenly the nature of the Fathers angels and the gods own outlook on the world made sense. Here in Heaven, the Archons had arranged themselves with the reality of the Sundering, having remade their home to survive in it's diminished state. Archons weren't working with and for Baator because they had fallen, but because they had no real choice. They accepting all the little evils to stave of the bigger ones, and in doing so had compromised themselves and their ideals. The local government was endorsing things such as slave trade and heavy punishments for criminals because they had prioritized the perpetuation of the system they had by ensuring Order over the morality of it.
The Father was doing the same. He had Lucan do not what was right, but what was important to ensure the Fathers control. In of itself, the Father would not be able to or even caring to build a better society, instead cementing the one they had as "good enough". Others, like the Maiden, would still try to improve things, but Lucan had to realize that his own role had been and always would be that of a divine soldier, propping up a regime without question. But he had questions and he had learned things in Heaven. While he tried to help the people there to his best ability, he learned about both the fallibility of gods in a very visceral way, but also that gods were not immutable and would change by the outlook of their followers on them.
To him, the Father has fallen, much as Heaven has. From the ideal he had of him as the firm but benevolent figure that guides humanity, he has become little more then a tyrant. Not malicious, but neither kind, all too willing to make sacrifices and accepting to turning a blind eye to many problems plaguing Westeros and the world in general. In Lucan's opinion, that is the result of reality of Westeros, the blind obedience asked for by the nobility, aided and abetted by a Faith that is deeply steeped in politics and unwilling to truly hold up the ideals it espouses.
So for him the dissonance between his belief that he was and should be fighting for a good cause and the reality of the Father resolved himself in a commitment to become the shining leader he wanted to be, not by being handed this grace from the gods, but by earning it from his own efforts. Thus, so he hopes, he can inspire the people to better themselves and ultimately even redeem the Father. This is why he tried to decentralize the Faith in the Conclave, to make it more inclusive and more critical of itself and it's teachings. So that people can question the injustices and failings, and better themselves and the Faith for it.
And this is why Lucan will never compromise with Viserys. Because he believes that the means taint the end, as evident by how far Heaven has fallen, no matter the good intentions of it's rulers. How both Heaven and the Father have become so engrossed in games of power and control that they lost sight of the cruelty and wrongness of their means. Lucan is no longer buying into the utilitarian argument that anything can be justified, were it just to cause enough Good down the line, because he realized that you can justify absolutely everything that way. Even Baator is a necessary evil if you compare it to Abaddon, the Abyss or the Void.
So he took it upon himself to fight this fight for the sake of an ideal, not cold calculations and slippery arguments. He took it upon himself to shoulder all this weight, because it is the right thing to, knowing full well that it might and likely will break him. Because that is the one sacrifice that a true Paladin is willing to make. Himself.
So Where I'm Going With All Of This
First off, the wandering adventurer and the inspiring leader are not the same thing. Danelle and her crew are the former, going around and solving particular problems, then skedaddling back home and doing it again somewhere else. Lucan is the latter, leading by example and trying to both reach the population at large with his message and to inspire them to solve their own problems and better themselves. Leadership is not something that just happens, but something that you need to put conscious effort into. You got to have both a desire to lead and a goal to lead towards, however good or bad that goal might be.
The second important point is to differentiate the character of Kreia and the arguments she makes into two possible interpretations.
On the one hand, Kreia is a teacher, who questions the actions of others to provoke them into analyzing them and their motives for performing them. This is motive associated with Darth Sion, who is a model of a person with neither morality nor philosophical insight, but this side of the character has less bearing on what I'm talking about, so we'll not dwell on it here.
In the other interpretation, you take these arguments at face value and assume no hidden motive in posing them. In this interpretation, Kreia is an amoral Devils Advocate. She questions both the point of kindness and cruelty, arguing that they are meaningless and their consequences not predictable or even run counter to the intent of the action. This is both a refutation of the idea of Utilitarianism that the morality of actions can be quantified by looking at their outcomes, since those outcomes are too difficult to predict to facilitate an informed choice, and an argument in favor of Nihilism, by implying that because only the actor himself and his motives matter, objective morality cannot exist and all actions must and should be selfish.
This argument and the connection between Utilitarianism and Nihilism, are personified in the character of Darth Nihilus. He is a personified Utility Monster, consuming resources (in his case lives) to sate his own hunger, yet he can never be filled. Yet the argument can be and is made that this is acceptable behavior, because if you assume that his happiness is weighing greater then the happiness of those he consumes, then consuming them becomes a moral act by the strictures of Utilitarianism. Since he is a nihilist, this is the case. As nothing objectively matters, his subjective experience is the only valid value of anything he does, thus consuming entire worlds to sate his own hunger is a morally acceptable act.
This illustrates the core vulnerability of Utilitarianism. It is not a philosophy that can be used to derive morality, just a mode of thought that can be used to judge the morality of actions in relation to an assumed goal. This is cloaked by the assumption that "maximizing happiness / utility" is some objective value, but it isn't. Happiness is deeply and utterly subjective measure, which' definition can vary tremendously between different persons. Thus, as Darth Nihilus demonstrates, Utilitarianism needs to have a preexisting moral framework to be applies to and it will never produce outcomes that are better or worse then those measures you originally applied.
The conclusion here is that Utilitarian argument is unable to answer the question about the morality of a person. Morality is not a numbers game, where 1,000 fed orphans weigh up one account of rape and murder. It's not a system that can be gamed, because the system is built on basic assumptions about morality, good and evil, thus it will always be a mirror of it's own foundations. By saying a fed orphan is +1 point and raping someone is -1,000 points, I'm just rating a judgement that was already made in an attempt to either extrapolate new judgement, or to justify actions to myself. If I do not think that feeding orphans is important, it has a value of 0. If I think feeding orphans is detrimental to them by making them rely on charity (an argument made by Kreia at one point), it suddenly gets a negative value.
Therefore, any and all arguments about scale of affected change, net outcomes, the Greater Good and ends justifying means are farcical. They are justifications for actions, not valid arguments about the morality of them. If you assume that torturing criminals is a valid way to prevent others from committing crimes, then a clean decapitation of a murderer is +100 points while disemboweling him and parading the screaming carcass through the streets is +1,000 points. From the same logic I could extrapolate that, since bad things happening to bad people is good and rape is really bad, that prison rape should be encouraged, because it's +500 points of utility by scaring others away from criminality.
Viserys actions are informed by the assumption that he has the power and thus the right to makes choices for others, that he is capable to rule the world and thus should, that he is wiser and knows what is best for others, and that he is the one and only chance to prevent the apocalypse. These are the basic assumptions of his character and all other acts are justified by the means of Utilitarianism, by assuming these core axioms to be correct and "good". These very same statements can be made about a host of totalitarian dictators, because Viserys is a totalitarian strongman. If you assume that there is a Jewish world conspiracy out to kill us all, then the Nazis are right. These ideologies work because they adherents buy into the core idea and then reason that their responding actions are justified. People join such movements or cults because they think it's easy to spot ludicrous ideas like parasitic alien souls for the nonsense they are, but these ideas are the results of far more believable base concepts they too might accept without much thinking.
The reason why these arguments always turn so toxic here is that many people feel personally attacked when these ideas are challenged, because they do buy into the image of the all-wise and all-powerful strongman, as long as that strongman rides in on the right mix of core axioms. They argue in favor of virtue springing from results achieved, because they are perfectly willing to compromise their morals in the pursuit of a goal they deem worthy.
This is the idea behind Yrael, who has all the good intentions, but can be reasoned to commit all kinds of atrocities to see them come to fruition.
The other side of the coin is Lucan, who is holding his values as sacrosanct, even in the face of hardship, death and failure, because sacrificing his morality means loosing it.
So, to answer you core question, yes, it would have been an option. The thread always wanted power, sometimes with good intentions, sometimes without, but it was easy to steer the character by appealing to this desire for power and throwing some Utilitarian justifications over it while selling the axiom that power is necessary, thus actions geared to amass power are moral.
Instead, Viserys could have just helped people without second thoughts. He could have freed the slaves and helped them build their own government, instead of inserting himself as their new unquestionable and almighty overlord. He could have gone the path Lucan did, accepting the mantle of Azor Ahai and trying to use this to reshape the religion by inspiring the faithful. He could have forgiven Robert Baratheon, Tywin Lannister and all those others that wronged him for the sake of building alliances against the threats of the world, instead of pursuing his own revenge on them. He could have engaged with other people as equals instead of as tools and minions to be used as his leisure, or enemies to be destroyed.
But we got a character poised to become the supreme, quasi-deity ruler of most of humanity, because this is where the desire for revenge, the lust for power and a stark "us vs. them"-mentality has led to.
TL;DR
Don't drink the Utilitarianism cool-aid. It leads to bad places.
If I had a choice to do everything the same I'd probably go for it. We've made great progress in conquering the plane.
