Shishio Is the Argument That Kenshin's Vow Should Fail
SPOILER!
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Same Government · Same Assassin Job · Opposite Conclusion
Kenshin and Shishio were both used as assassins by the movement that created the Meiji government. Kenshin responds by refusing to kill again. Shishio responds by deciding that the government proved strength is the only honest law.
Why it matters
Their final fight is not "good assassin versus evil assassin." Shishio is the version of Kenshin's history that accepts every ugly lesson the government taught them and refuses the possibility that anyone has an obligation to become better than the system that made them.