To be fair, right now I can't feel any empathy to Kouki. He was painted in a bad way from Hajime's perspective from the start, and it's easy to see why... I think I need to read the full after-story to understand his Redemption arc.
Bc unless im wrong, the dude wasnt a bully but just a "normal ikemen" (pre-isekai)
and in isekai, wasnt a "expel from party" cliche guy (trend didnt exist yet) but just a normal highschool boy
And in story, from hearsays, he basically didnt betray MC/group/humanity, but basically did a bad decision that he thought was "GOOD" but not
That is why author didnt write MC kill him, unlike "evil classmates"
So, if you read from begin of afterstories,
you basically sometimes hear classmates mention : Kouki quit school voluntarily. Helping people around the world. To redeem himself.
Its NOT bc he isnt forgiven or anything (classmates/kingdom forgave him) but because HIMSELF cant forgive himself of what he did. (very profound thing)
Heck you could even make comparison to Emiya (Fate IP) to him.
And this particular arc, is basically the final redemption of himself, where he save people(what he couldnt in isekai1), and finally do the hero role that was expected of him,
but most importantly, saved himself.
Forgiving himself of his crime he did. Forgive his incompetence. And no longer remorse on it.
If you read the early chapters of this arc that i shared.
You can see that Kouki basically trembled before the fight began. He tried to smile to recomfort the people, while his hand trembled. It basically showcase that he, Kouki, isnt a hero, or character in a story, but a flesh and blood, normal human being, that love, hate, fear.
Just.
A normal human that fear death, like you and me.
The numbers of enemies was basically innumerable. And basically is the sole fighting force available to this town. With no reinforcements until far too late.
He could simply "leave alone" but didnt.
Despite knowing full well, with a 99,99999% probability he will likely die
(You can even say, that he wanted to die to redeem his previous error (in main plot))
That's because of the Cookie fact, isn't it?