Satsuriku no Tenshi

Synopsis

Realizing that he will need Rachel's help to escape the building, the man introduces himself as Zack and agrees to kill Rachel once they escape. They both travel up to B4 where they find a graveyard, which includes a gravestone with Rachel's name on it. They both split up to investigate the area, with Rachel finding a library while Zack destroys many of the gravestones out of boredom. Rachel finds notes promising her that the writer knows her true wish, as well as files documenting other people who have been in the building, including her and Zack, whose real name is Isaac Foster. With Zack's help, Rachel is able to unlock a door to proceed further into the building while Zack is approached by a masked boy who is angry at him for destroying the gravestones.

(Source: Wikipedia)