In the prophecy for Cersei, so far everything has come true. Therefore if she is to be killed by the "valonqar" , and valonqar is high valyrian for little brother, Cersei will be killed by a little brother. Yet it does not say her little brother, it says THE little brother. The Hound will kill Cersei, going through the mountain of course, but he will be the one. That is his purpose, being the ultimate little brother.