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Perhaps most telling is that the phrase is bad Classical Chinese. If I were to put it in English as "Evil, namely slay," that is not perfectly correct but would give a sense of the mangled grammar. The Ruroni Kenshin catchphrase is an uneducated riff on Chinese philosophical statements like 心即理 which are quite subtle in meaning and misunderstood by many modern Japanese people.
There are no Google Books results for 悪即斬 (that is, "Aku Soku Zan") in the 20th century. Google Books has limited coverage of Japan before 1990 but at least something ought to come up. There are also multiple Japanese Yahoo Answers posts saying that the author of Ruroni Kenshin made this up.
There are not many historical sources for Hajime Saito, as it is said he was a man of few words.
Upvote:-2
Both sides of the Meiji Revolution in 1868 Japan (the Shogun and the Imperial Court) used "aku soku san" as a motto and a way of life, it was a radical sense of justice that they held and was believed to be the only way by samurai of that time. the anime about Kenshin Himura is based in a lot of fact and even most of the attacks (the ones the aren't humanly impossible, of course) are real including the aku soku zan. But it wasn't exclusive to the Shinsengumi