To me the story is about corruption, its nature and its cost.
Each leader in the story is offered power by a spirit whose only wish is destruction. Each accepts; both, therefore, are corrupted by their yearning for greatness-- to live forever, ever-prosperous, victors in every battle.
What is interesting to me is that the first to acknowledge the leaders’ corruption seem to be the leaders themselves, who halt their immediate efforts at mutual fratricide and lay their knives on the table. Each sees in the other a mirror. In that moment, they seem once more kindred-- but, alas, only to each other.
The clan cannot stand to be guided by the corrupted, and so disbands, so whatever wisdom these two sinners may have learned though their mutual corruption goes to waste. So, even one who repents corruption will not be trusted again easily, even if through the experience they might have become more worthy of leadership.
A thing that strikes me: had either, at that time, have said “no” to the bargain, that one would have presented no mirror to the corrupted sibling-- only a victim. And likely the blow would have fallen.
Perhaps-- probably, if human history is any guide-- the clan (or at least the leader’s own faction) would have accepted that leader, and excused the corruption. All the more if the spirit fulfilled its bargain.
Maybe it is easier to follow a villain than one who is ashamed.
Yet, also, it is the clan’s own conflict-riven hearts that summoned or else birthed this demon, so perhaps the really culpable party in this tale is not the leaders, tempted by power but ultimately repentant, or the demon, which only behaves as its nature dictates, but the clan members themselves.
They, too, could have felt shame over having created this demon that corrupted those who led them, and the clan might have been saved. But instead they turn away in disgust. They cannot join their leaders in their shame over what they had brought about, over the corruption of their own hearts, and so disburse, bonds of affection destroyed, the demon victorious.
So I guess if it’s about corruption, it’s not so much about the corruption of the great, of leaders, but the corruption of the small, by bitterness, self-righteousness, and ill-feeling, which then goes on to tempt and corrupt the great. It’s in the hearts of neighbors and kin that conflict is birthed-- the same neighbors and kin who will turn away in disgust from what their hatred brings.
In the end, then, it’s a warning against harboring bitterness and spite in the only hearts any of us really have control over: our own.