Zetsubou Shouraku

In a certain town of a certain prefecture, Misawa, a high school boy, lives a dull unchanging life. He goes to school, and then spends his free time afterschool at the nearby shopping mall. He thought such days would go on forever. But then, his older sister gets roughed up and is taken to the hospital. Meanwhile, Tsuruta, a high school girl who goes out hunting with her grandfather, notices something strange in the mountains.... These are just the beginnings of the horrors that Misawa, Tsuruta, and many others would soon face....... Be killed or be raped, the worst kind of monster is here!

(Source: Kodansha, translated)