"Everyone who isn't us is an enemy."
Cersei Lannister lives by this creed, and it is both her greatest strength and weakness. While she is power-hungry and trusts no one outside her immediate family, she is neither overly smart nor politically astute. Despite the danger of discovery by her husband the King, she continued an incestuous relationship with her brother, Jamie. She bore him three children: Joffrey, Myrcella, and Tommen.
When King Robert died, she turned her political wiles to the benefit of her House, fighting against Joffrey's enemies during the War of the Five Kings. For all her ruthlessness, Cersei genuinely loves her children. Each of their deaths exposes her weakness, driving her mad in her quest for revenge.