One thing I didn't get... (spoilers)
Maybe I missed something, but it didn't make any sense to me why Peter (knowing he's not the killer) would be literally busting through the basement window calling for his girlfriend like that?
Only thing I could think of was - he's been watching her from outside and saw she ran into the basement? But if so, than he would of seen the killer (Billy) chasing her also, right? Would be kinda cool/creepy and make total sense if he was in on it with, Billy... But guessing it's just a goof though, because they were wanting viewers to think it was Peter until the end. Any other ideas?