Tsumi to Batsu: Hoshi no Keishousha

Ruffians. It's been a little over two months since these monsters began attacking humanity. Japan, or rather the entire world, was now facing an unprecedented crisis. Originally created as "food" for a planet-wide famine, these monsters now travel south through the archipelago as they hunt humans. Along with many people, the main character, Saki, also escapes to the south, but on the way, he is attacked by a Ruffian and seriously injured. Saki is then saved by the blood transfusion of a mysterious girl named Achi, the leader of the "salvation group" who calls herself a "saint". Saki then joins the group and learns about the sins and punishments of humans while fighting against the enemy of humanity, the Ruffians.