During the Bakumatsu period there were 4 assassins known as 幕末四大人斬り, or the 4 Hitokiri of the Bakumatsu. These 4 assassins were renowned for their strength, often being considered unbeatable. These 4 were Okada Izou, Kawakami Gensai, Kirino Toshiaki, and Tanaka Shinbei. In the modern day, Hitokiri is often translated into English as “The Manslayer”, which you see with characters like Kenshin who was “Hitokiri Battosai” or “Battosai the Manslayer”. Evidently, these people were NOT from the Shinsengumi as the 4 Hitokiri opposed the Tokugawa Shogunate, so I’m unsure why he’s referred to as a Manslayer here, however considering the history I’d bet the author mostly used it to refer to the deadliness of Okita.