A group is held by a madman who is killing them one by one until they identify the traitors securing their freedom.
It is a simple contained horror. There is no real main character. As the audience begins to think they know who is the protagonist, I kill that character. Risky, but means it is completely unpredictable.
I need help tightening the logline.
PS you never know why they are selected or who the madman is. Just people in a cage.