Unbowed, Unbent, Unbroken - S5-E6
Continuity mistake: When the Sand Snakes attempt to kidnap Myrcella, as Nymeria Sand runs up to Myrcella to seize her, she is holding her large whip in her right hand. However, in the very next shot, the whip has disappeared, and Nymeria is now holding a dagger in her hand instead.
Answer: Basically it's because he broke his vow. A member of the Kingsguard is sworn to protect the king at all costs. Jaime elaborates more on the deed to both Catelyn Stark and Brienne of Tarth, telling Catelyn that no matter what course of action he took, he would be breaking one vow or another (i.e. if he obeyed the king, he would conversely be disobeying his own father), and telling Brienne that the Mad King was planning on burning all of King's Landing, but he did not bother to tell Ned Stark that. Ned Stark felt that killing Aerys was dishonorable and excessive. Robert Baratheon still could have usurped the throne without Aerys being killed.
Phaneron ★