Gekka no Hazure Gedou

In this world, there are creatures called Gedou, which are murderous crow monsters that come from fallen humans. Since their king was sealed away, they become active only at night, and humanity is able to achieve a daytime-only kind of normal life. A selfish young brat by the name of Shiramine Miroku lives with his little sister Hifumi and mother, spending his time constantly whining about his mother's attempts to get him to behave and be useful. One day, he accidentally frees the king of the Gedou, sacrificing his mother's life and both his and Hifumi's humanity in the process. Now he is a Gedou, and he has joined an anti-Gedou team called the Rikudou. He is determined to find a way to turn Hifumi, who is now an Oni, back into a human. In the meantime, the Gedou king is on the move, and he also has an interest in Hifumi! (Source: MangaHelpers)