The night world

How would your life be, if you had to kill monsters every night, even though yourself are a much worse monster than that??. thats the life 5 young teens have, and even more have. They work under a group, you could even call it a organization, there kills low level monsters, or as they call them "weaklings". But what if i tell you that, all them in this group or organization, all are monsters too?? they have always been it, not like the monsters they kill. But that will happen for our young teens when a new enemy comes, and that will our young heroine do, when this people show her a new "side" of they things they do??.