Aku Zoku Zan
Kill Evil Instantly is what it roughly translates into plain English. It is a motto that united soldiers in late 19th century Japan, during the Meiji Revolution period.
Though some may consider such a "law" an extreme measure, those who followed it did so religiously. Believing that no matter what side they fought for, that as long as they upheld Aku Zoku Zan, they would accomplish their mission.
In Fiction
Aku Zoku Zan is used in the anime Rurouni Kenshin. It is the 'brand of justice' that becomes Hajime Saito's trademark and that he upholds to an absolute power. Though Hajime Saito pronounces it "Ak Zok Zan", it is a mispell of Aku Zoku Zan. He believes that anyone guilty of corruption, or those that would bring harm to the people of Japan and the nation itself, should be slain. With his trusty katana, whom he says that even without a name it has saved countless times and he trusts its above all, and Shinsengumi's Gatotsu, he is more than ready to deliver, as shown with Uonoma Usui, of the Juppongatana.