Cersei Lannister is one of the most disturbing, heartless, cruel characters in A Song of Ice and Fire. Her character deteriorates and slips into insanity just as Aerys Targaryan did. She is filled with nothing but contempt for the world, and desires only her own power and control. She repeatedly justifies herself by saying a mother will do anything for her children but she often appears to be a self interested mother as opposed to someone like Catelyn Stark who is a self sacrificing one. Cersei goes against the fundamental definition of motherhood, which is self-denial for the sake of your children. This can be seen when Cersei slaps Tommen, and often threatens to have a boy beaten while Tommen must watch. It can also be seen by the fact that she had her first child killed, since the child's father was Robert Baratheon and not Jamie, her twin and lover. She shows disinterest in everyone, including her own children. She is manipulative and uncaring.
A huge factor in the decline of Cersei Lannister is the prophecy she receives from Maggy the Frog. She calls it a curse: "Maggy's curse bloomed like some poisonous flower." (A Feast for Crows, 515). This prophecy eats away at her as she slips into paranoia in the knowledge that her future will never be secure: "'Queen you shall be...until there comes another, younger and more beautiful, to cast you down and take all that you hold dear...Gold shall be [your children's] crowns and gold their shroads...And when your tears have drowned you, the Valonqar shall wrap his hands about your pale white throat and choke the life from you.'" (A Feast for Crows, 771). She knows she is going to lose her position of power so she becomes paranoid to keep it at any costs. Yet she is often filled with self pride to the point where she is blinded to the possibility that her plans could fail. Her downfall comes through arrogance and conceit, but it also comes from the prophecy she received when she was young.
Cersei's decisions are often made as this prophecy echos in her head. She loathes Margaery Tyrell since she believes that she is the younger, fairer queen that will take her place. This loathing leads her to create her downfall by any means necessary, and her hastiness traps her. Cersei created the very monster that will consume her. She allowed the High Septon to create an army separate from the crown: "'The Faith Militant reborn...The Warrior would lift his shining sword again adn cleanse this sinful realm of all its evil.'" (A Feast for Crows, 602). and with this army the High Septon imprisoned both Cersei and Margaery for infidelity. Cersei was eager to accuse Margaery of being wanton, but failed to realize that she herself was guilty of the same crime. To Cersei it is everyone's fault but her own.
The last piece of the prophecy describes the Valonqar. This is the part that Cersei most readily misunderstood. Valonqar means little brother. When Cersei received this prophecy her little brother Tyrion was alive and easy to hate. He was hideous in her eyes and she loathed him for it. She put up walls between them and never trusted him but always loathed him. She was sure he would be her undoing. But she failed to realize that she has two younger brothers, since Jamie was born after her: "'We are one person in two bodies. We shared a womb together. He came into this world holding my foot." (A Game of Thrones, 485). She trusted Jamie because he was beautiful, because he was like her. As she becomes more and more self interested she failed to realize that she hurt Jamie with her unfaithfulness, and Jamie in response grows to loath and mistrust her. She is undone by a little brother, but with prophecies it is never the one you expect.