Wish I'd had a little more time to flesh this out further (particularly at the end), but I'll post what I have before it becomes meaningless.
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Creation is not kind to humanity. This fact underlies all possible rationale for this decision, but thus far has lain fallow, an unspoken truism. But the fact of the matter is that it is only unkind because human survival on anything beyond the broadest populations levels is an afterthought. Humanity were conceived and forged as tools, engines of worship to fuel the power of gods and daeva alike. Their sapience, their emotions, their will exists only so that they are more efficient at this task in service of their betters, and they are purposefully designed without the ability to rise above this position. It is of no concern to the alien minds that held and still hold the tools of Creation whether the beauty of acid streams and flensing winds are apparent to human conciousness as their flesh melts and their skin is flayed. They are in fact designed for the contrary, to fear the myriad features of this greater world so that they turn to the superstitions that feed their function as one feeds a fire. So long as this quota is met, the tools function well and one cares for the well-being of a tool only insofar that it adequately serves its purpose. An individual human is less than chaff, less than a thought.
Odyssial has understood the position of his race, on at least the most gross terms if not all the esoteric details (though the introductory piece indicates he knows the majority of these as well), for the entirety of his life. He will have grown while hearing of the fall of tribe after city after civilization, in some cases due to malice but more generally just as a fact of life, no more predictable than the weather and with as little explanation. He would not possibly be a stranger to the loss of kin and friends or even just the familiar stranger that frequents a certain park until he one day disappears. It is simply the way of the world, as unchanging as the Sun. The pain and struggle of life in Creation is familiar to this man and he, his culture, has internalized it so that they might best game the system as it were.
He has worked the majority of his life to meet this goal, to become all that is humanly possible, in search of security for those he holds dear and to create a foundation for more, for a rise beyond the means and the unconscious prospects imposed by their creators. He has met with the Daeva, learned what small portion he can comprehend of their ways, walked among them as more than an insect to be unknowingly crushed, even if only rising to the position of a minor animal that crosses the path of those so much greater. But it has never been enough. And in a single year, all he has known is gone. His home is crushed, his people torn from him one by one.
Assuredly, Odyssial feels fury. How can he not. The corpse of his last family lies before him and the murderer speaks of winning a gamble. This monstrosity seeks to manipulate him, to impose the most extreme guilt, merely for its own amusement, to beat the odds and accrue greater reward. He is a game, his sister was a game, his family was a game, his entire race are a game. The hate he must feel, to be the lifelong toys of uncaring masters who barely deem to acknowledge that humans exist beyond the needs of god and daeva. Antipathy will drive his blade harder, push him to end all those responsible. Yes, anger will form a great and most terrible resolve to end those Odyssial perceives his enemy. "Let them suffer the fury of his thousandfold vengeance, and pray he does not ask more." Yet what is this more? What could unceasing anger drive a man of Odyssial's will to do?
Moreover, what if that should rage ever collapse? If even the Sun can die, falling behind the horizon, what hopes have the sentiment of a single man? Humans are designed for fear, to experience terror upon terror so that they might more frequently and ably plead and implore the divine. And so, emotion is fleeting, a 'feature' allowing recurrent prayer. Anger too then, no matter how acrid and heartfelt, must eventually fade with the passage of time, leaving only cold ashes and a goal without foundation. Perhaps Odyssial will be able to sustain the burning rage at his core through the destruction of all his enemies, perhaps not. Emotions are unpredictable in that manner. But should he succeed, what then? With all the verve and spirit gone along with his agenda, will he allow himself to collapse a hollow monument to death and vengeance? It is a possibility I suppose, but not a particularly likely one, as driven as Odyssial is and shall be. It must then be considered the effect of century upon century of unending enmity, the self-reinforcing ire that stands Odyssial through his years of war. He would have stoked the flame of his rage day after day with each new or repeated atrocity, each barbarity, intensifying the fire within until it becomes part of him. Can he survive without that buoying passion or will he seek out some new vendetta to rekindle the spark that has fueled him across the centuries? His passion would then become self-consuming, requiring new and greater fuel to avoid growing cold and empty, devoid of the purpose that is his life. Further, in and of itself, fury can only be destructive, it cannot in itself build, only tear down. He has lived seeking out enemy after enemy, leaving their corpses behind him and also the burned fields and torn homes. When becoming history's greatest killer is in and of itself his goal, the aftermath can only end in death. And death is an everyday occurrence in Creation, so common as to be expected. Nio, his dear sister, achieved his survival not through prayer or divine intervention but through effort and study in a sorcerous talisman manufactured of her own hands and mind. Knowing she would die, she chose to save all that she could. Sacrifice such as this should be honored, yet to fixate upon it is no honor but instead condemnation. Forging a path of death purely for the sake of death cannot blaze new ground. No, it is not wrong to care, but expending all his effort in service of a single emotion can only but blind Odyssial to the alternative possibilities.
Assuredly, Odyssial feels guilt. How can he not. Even if he knows the logic is unsound, it is human to assume some measure of responsibility. It was his choice to walk among the Daeva, to become known to them, to draw their interest, to rise above the invisible insects that litter the world broken and unlamented by any except their fellows. But yet that is shallow reasoning - the appearance of Lethos was entirely beyond his control. His striving could not possibly have contributed to its coming and the schemes of Larsyifex derive from that shatterpoint. Yes, he may have walked among the daeva uncovered and unmasked, but perhaps the conceived charades would have merely provoked an earlier response and doomed himself without recourse? And were this to have passed, who would have stood in defense of those few survivors and ensured their lives to this juncture? Even without this daeva's passion for gaming, there is not, cannot, be a guarantee that they would have passed without encounter the myriad other daeva that litter this world.
No, no matter his regrets, the past cannot be altered. This is a fundamental rule above all others, time is immutable. The dead will remain so, and no amount of effort, nor monstrous will, can countermand this eventuality. This event, as all things, is a lesson, a tutelage that he will not soon forget, but to dwell upon how things might have been is both to dishonor the memory of the fallen and to abandon the opportunity held in the future. The tears must burn in his eyes and the pain cloy his throat, but a defeat, however monumental, is no reason to accept that things must always remain thus, to be bound by a failure to accept weakness in purpose and spirit. Odyssial is not beholden to false pride - he acknowledges his faults and works endlessly to expunge them - and it is hubris to accept responsibility for the outcomes of all possible choice. So far as his actions have in part contributed to this outcome, he will reflect and he will emerge with a greater appreciation for consequences. After all, once burned, twice shy. But to accept the possibility of limitations is to acknowledge the inevitability of defeat; to allow the prospect of weakness is to allow excuses for failure. After all, some things are beyond him. It is understandable that he cannot stand against that foe, overcome that mountain. Yet no man can stand when he betrays himself and that path would be the greatest betrayal possible, for it would sacrifice the chance of success on the altar of bitter contrition. Would he have succeeded if he had tried the slighest bit more, worked the slightest bit harder? These questions would haunt him, and each such thought would dishonor the deaths that have come previously, as these would be an anchor upon Odyssial's soul pulling him down to guilt-ridden resentment. Better to have toiled until his hands bled and his bones cracked, so that he knows he has worked his best, surpassed all proclivity for debility, so that he might stand at his best and greatest even at the moment of his impending death, to strive ever higher and farther beyond all measure. No one is served if Odyssial is burdened by that most terrible of questions, "What if I had?" For in truth, this world is made for the daeva and the gods and those like them. It is not that they are too strong, though this is true, but rather that Odyssial and those he has lost are too weak.
Before all else must come survival. Nothing can be achieved if death ends Odyssial's ambition before it has a chance to enact his will. Thus, his rage, his guilt, must be turned to this higher purpose, for upon the crux of Odyssial's life rests all the potential of his existence. To that end, the truth of this world must be acknowledged, must be examined, must be dissected if there is to be any chance of overcoming it. But Odyssial has not yet reached that stature; though he may bend the rules, he cannot break them. Idealism is insufficient and delusion will lead him astray; weakness cannot be afforded if he is to seek an end to the status quo. And so he will play by the rules of the Lords of Creation because ultimately, humans are made to be weak - it is their purpose so that they have reason to fear, reason to pray. It is not their fault and no blame lies upon them for this, but still the fact remains. The innocent perish and the kind suffer. And so they die, by the droves and the hundreds and the thousands, unremarked and undignified. Death is the fate of all humans, and he cannot change the past. Yet still Odyssial will tread forward, stripping himself of weakness, of infirmity, so that upon the next day of crisis he will not be found wanting. Reality is cold, and so too will he be cold. He will seek out every advantage, labor for every strength, ponder every action, strip himself of all that might diminish his will and limit his reach. As he cannot depend upon others to succeed, he must discover every chance for himself. He will survive until that day when the edicts of reality bow to his will. Until this world permits sensitivity without punishment. In the end, this world obeys only power and only while living can he seek it. Thus Odyssial will find a way. He can do no less and still be Odyssial.
Edit: Added a few lines and corrected a few incomplete areas.