I don't know what should I say other then I agree with one of the word that mc say, "I choose to ignore common sense". That why I also choose to ignore common sense.. . Let just laugh of this masterpieces common sense manga.
Given that seppoku involves death via disemboweling oneself if not done in such a way to cut a major artery and to die from blood loss, just the regular form could be tortuous.
Scoopoku with such rough cuts would be true torture unless done by a mining professional. Good thing he stopped her.
Forgive me my melodrama, this manga just deserves it though :)
In greater seriousness, this is oddly the first manga to inspire me the read a bit about the history of seppoku and the Kamikaze pilots that extended it in a way during WW2