This is the theory I was referring to
@TheKnightOfTheSea
Shin and Mou Ten will be appointed 6GGs first. This will greatly upset Ou Hon for obvious reasons.
Around the same time, Yan’s botched assassination of Sei will result in Kou or Shou Bun Kun (or both) dying to protect him in identical fashion to Shi Ka.
Reliving this traumatic experience of grief and powerlessness will change Sei for the worse and begin his descent into tyranny.
Shin will be appointed Supreme Commander to invade Yan. Though concerned by the change in Sei’s demeanor, he will rationalise that avenging Hyou gave him the closure he needed and convince himself the old Sei will return after he crushes Yan.
The Yan Campaign will be a brutal affair, and by the end of it Shin will feel all types of ways. He will be especially disturbed by Qin generals and soldiers under him presuming to flaunt the laws of war and committing atrocities. Naturally, he will adjudicate the king's justice and imprison the offenders (and we may even see him carry out wartime executions himself).
The successful Yan Campaign will not bring the old Sei back. He is changed for the worse and their relationship begins to strain as a result. The two are at odds almost every time they meet in person and things come to ahead at the end of the Wei Campaign led by a one-armed Tou.
While Shin and Gai Mou settle their affairs in a bloody duel, Ou Hon, calloused by shame and frustration and possibly further egged on by Ou Sen's comments, diverts the Yellow River to prove himself as capable. The flood kills more than 100,000 people, including tens of thousands of civilians.
An enraged Shin will have to be restrained from killing him, there may even be a brief skirmish between the HSU and Gyoku Hou (that is of course swept under the rug), prompted by their respective generals getting into violent confrontation.
After they return to Kanyou, Shin expects justice only for Sei to thank and praise Ou Hon for ending the war quickly and without any further losses to Qin soldiers.
After an angry shouting match in which Shin accuses Sei of hypocrisy and creating another Kan Ki, he leaves the Kanyou in a fury. Sei will be extremely angry with him but likely take it out on court advisors urging him to have his former best friend executed.
Hara will use text boxes to reveal Shin and Sei never reconcile their differences and that the king losing his only true friend and ally is what plants the seeds of calamity for the Qin Dynasty.
Tou, feeling responsible, retires from the 6GG. He is rewarded with governorship of Han. Ou Hon is elevated to the 6GGs and becomes Sei's new guy,
thereby giving Hara an organic justification as to why Ou Hon/Wang Ben was much more prolific than the comparatively obscure Ri Shin/Li Xin.
By the time the Chu Campaign comes about, partly motivated to extend an olive branch and make peace, Sei sides with and appoints Shin to lead it over Ou Sen. After it fails, he doesn't punish Shin despite having justification, but the atrocities continue unabated everywhere Shin is not and Sei remains beyond persuasion, and so the two never reconcile.
After unification, Shin retires and is given titles, honours and a sizeable fief (Longxi, modern Gansu) for his accomplishments.
In truth, however, it will also be a
soft banishment as the territory he is given will be far removed from Kanyou. When the Qin Dynasty falls,
news won't reach Shin until it's too late, but
it wouldn't have made a difference as he has put down his arms for good. After a lifetime of war and strife, and now likely a widow, Shin has no further appetite for bloodshed.
The ending of Kingdom is bittersweet, but Hara will reveal that Shin's descendants do go on to be impressive individuals and the eventual royal house of the Tang Dynasty. That is the end.
