Kamisama, Kisama wo Koroshitai.

After losing her entire family to a combination of tragedies deliberately caused by certain people, a girl decides to take her own life. Before she has the chance, though, she witnesses a murder and is subsequently cornered by the perpetrator. In search of a greater challenge, he proposes to help her to kill the people who led her family to death… so long as she agrees to allow herself to be killed by him in the end.