The Soundtrack *SPOILERS*
I will state up front that I missed the first ten minutes of the movie, so perhaps what I'm about to say is more fully explained in the beginning. Feel free to let me know.
I've read several reviews that think the use of a soundtrack for this "found footage" movie is out of place. It is my understanding of the ending that the killer takes the footage he has been shooting and edits in the footage of the five kids to create a film. So he added some music. It might catch you off guard while you are watching it for the first time, but I thought the ending made it clear who added the music in.
Also, the scene of his editing room doesn't necessarily break the fourth wall. He's shooting himself editing the footage before he makes the final cut.
I'll tell you who it was. It was that darn Sasquatch!