#theres something up. but then that gets into modern japanese history. which (chinese diaspora moment) is a whole other can of worms.

[reductionisms]

by what point in gintama had sorachi decided gin killed shouyou?

it's helpful to look at history here. shouyou's real life counterpart yoshida shouin was, in actuality, arrested in an anti loyalist purge and beheaded. he also really did run shouka sonjuku, of which rl takasugi and katsura were both loyal students.

unlike shouyou, though, shouin was extremely patriotic/ethnonationalistic. while the eng article doesnt talk much about it, shouin formulated the idea of, and advocated for, japan conquering and colonizing china, taiwan, the philippines, the ryukyu islands (now okinawa) and hokkaido, among other things. this proposal, while ostensibly to counter western imperialism, was founded in a fundamental and violent racism. its results were devastating. i will expand on gintama and actual japanese history further sometime soon.

in any case, the existence of katsura and takasugi in gintama already presupposes that they were devoted students of shou(you)in, and, furthermore, that shouyou was beheaded by the bakufu in the ansei purge (this is basic history you learn in school). since sorachi had gin as jouishishi in bakumatsu era from the very outset, it's safe to assume he also planned on introducing takasugi/katsura from the beginning, and therefore the concept of shouyou is loosely associated to gintama from its conception.

but when gin's past is introduced via katsura (ch. 5), the essential "trauma" is the death of gin and zura's shared comrades in the joui war. this is repeated in harasume arc (gin's nightmare), and even takasugi's introduction (can't you hear the voices of our dead comrades?). if sorachi had 'known' that gin killed shouyou from the beginning-- or, to be even simpler, if he had simply associated gin with shouyou from the beginning in any way-- i feel like gin's war flashbacks/thoughts would be a little more directed.

further, unlike zura and takasugi, gin isn't a historical figure, he's a mythical figure, from almost 700 years earlier. so he has no reason to be connected to shouyou in the first place, aside from the tangential relation of knowing his students.

that said, i think sorachi definitely knew gin killed shouyou by the benizakura arc (ch. 86-97). obviously here it's established that gin was shouyou's student, but, more importantly, in response to takasugi talking about how he can't help but fight with/hate the world that took shouyou from them, we get zura's "takasugi, i don't know how many times i've thought to raze this world into nothing, but he.. he's withstanding it. gintoki, the one who should hate this world more than anyone else, is withstanding it..." So takasugi and zura reflect on how shouyou (again, the well-known historic "martyr" for the meiji revolution) was taken from them, and the one who should hate the world the most because of that is gintoki. gintoki and shouyou are finally associated; gintoki's "hatred" is given primacy against even that of shouyou's real-life student's. yeah, sorachi knew by here.

but did sorachi know earlier? i mean, why not? just because gintoki's "tragic backstory" before benizakura is centered exclusively around dead comrades, doesn't mean sorachi hadn't decided he'd killed shouyou. i guess i just dont think sorachi was is good at planning, or writing in general, but that's my own issue.

an interesting argument could be made for the rengokukan arc (ch. 42-44). here the gintama-canon shouka sonjuku parallels are ridiculous. a mass-murderer-- literally referred to as a "demon"-- wants to atone, so takes up residence in an abandoned buddhist temple and adopts poor orphans who call him sensei. he and gin sit and talk on the temple porch almost like they're in a rakuyou flashback. gin calls him a human. when he attempts to escape with his kids in the night, he's killed by a shady government organization, and when gin learns of his death, we only see gin's back, a bit of his shaded face (a technique sorachi uses later on whenever gin gets upset about shouyou).

maybe at this point the gin-shouyou backstory was cooking in sorachi's brain. maybe some ideas of utsuro as well-- though probably not utsuro in the form we actually see him, perhaps something a bit simpler. notably, though, the demon/human, doushin, is killed by the bakufu: there's no moral dilemma, no teacher-slaying. it's not as direct as the shinigami arc will later be.

anyways, there was no real point to this, other than that it bothers me. i have no desire to dig through internet archives for sorachi interviews, either, so this is pure conjecture. if anyone has any other ideas i'd be interested to hear.