When Cersei was young, she visits a fortune teller named Maggy the Frog these scenes look set to be shown during season five, according to the casting call that went out. At that meeting she is told she would marry the king (and that he would father far more children than she would bare) and that her little brother would kill her. This set in motion her path and her belief that Tyrion would eventually kill her meaning that she took steps to ensure that this wouldnt happen. The fan theories are that this was totally incorrect because Cersei has two little brothers, as there is also Jaime to consider. There is a scene coming up towards the end of season four where Tyrion will kill Shae by strangling her to death with a chain of golden hands, which fans have interpreted to be a reference to Cerseis eventual end by being strangled by Jaimes golden hand. There are other fans who decree this to be entirely too obvious by the standard of Game of Thrones theories. They suggest that it may in fact be a double bluff and that Tyrion will be the one to kill his sister after all, while those who support the Jaime idea instead argue that Tyrions reaction to his double murder (coming up during season four) will prevent him from ever considering killing his sister. Who knows really? The one thing that they agree on is that Cersei is going to come to an untimely end at some point.
