In Rurouni Kenshin, Hajime Saitoh had a philosophy of life or something like that called "aku soku zan" that was something like "kill the evil immediately" which he mentions when fighting Makoto Shishio. Now in Dragon Ball Super Toppo says to Goku the same "aku soku zan". What is this really? Is this something from animes or the history of Japan or something?
I didn't find any good evidence at all for where this phrase comes from. A few weird Shinsengumi fan sites and Urban Dictionary claim that it was a real historical phrase, but Wikipedia and the question I posted to History SE claim it was not a real phrase and was invented by Nobuhiro Watsuki for the manga. Wikipedia claims without citation:
The "Aku Soku Zan" motto he lives by (悪即斬, most literally, "Kill those who are evil immediately,", translated as "Slay Evil Immediately" in the English dub and as "Swift Death to Evil" in the VIZ manga) is most likely fictitious, though it does encompass a common sentiment of the Shinsengumi during the Bakumatsu.
The only semi-reliable source among these is the Google Books search that Avery did for the History SE answer, which turned up no results for the phrase anywhere in the 20th Century. The History SE answer also points out that the phrase smacks of the kind of bad Classical Chinese that a non-expert modern Japanese speaker would come up with, which is at least decent circumstantial evidence.
All I really have to go on at this point are instincts, which tell me that if it's this hard to find any decent historical source on this phrase, it's highly unlikely that a mangaka doing a weekly action series for Shounen Jump would have managed to dig it up, so it was probably a fabrication. I don't know whether it was Watsuki's specifically or if it was one of the weird fabricated "facts" that sometimes show up in happy fun-time history picture books aimed at grade schoolers that you buy in the gift shop at the museum, but my gut says someone made this up in the modern day.
This would mean that when other anime use the phrase, they're referencing Rurouni Kenshin. They might be doing so intentionally; several high-profile manga creators of today, including Eiichiro Oda of One Piece and Hiroyuki Takei of Shaman King worked as assistants for Watsuki, and the series has remained popular, leading to a live action film released in 2012, with a sequel in 2014, so it's still in the public consciousness. It's also possible that after the phrase was introduced in Rurouni Kenshin, it became part of folk history, the way American films like Titanic have influenced the public's view of the historical events.