OK, everyone seems to be confused about this movie so let me explain it for everyone. The killer has a multiple personality syndrome, and was being treated in a psychiatric home. They have tapes of him portraying many different personalities, and they believe that one of his many personalities is the killer. The reason everyone has the same birthday is that every person is really him, all of these characters are made up in this guys mind. The way they figure they can cure him of his multiple personalities is to have them all come together inside of his head and hopefully they will all be killed off, with the risk that the killer will be one of the personalities killed. Let me repeat, the point of that motel, is to have all the personalities come together hoping that they will all die except for one, that one takes control of his life. As each character dies, that is one less personality he has left. In the very end they think he is OK because all of his personalities are dead, except for the girl. But really that kid is still alive, and he kills Amanda Peet. The exercise that they tried to do was complete, all the personalities died inside of his head, unfortunately the only personality that survived was the one that was the killer. I hope that clears it up a little bit.