The Prince of Winterfell - S2-E8
Continuity mistake: In the scene where Tyrion and Cersei are talking about sending Joffrey to the battlefield, Tyrion removes his cloth napkin from his shirt twice. (01:18:50)
The Old Gods and the New - S2-E6
Continuity mistake: When Maester Luwin walks towards Theon to try and talk him out of executing Ser Rodrik, he places his right hand on Theon's left shoulder. When the shot changes, his hand is no longer on Theon's shoulder.
Continuity mistake: When Tyrion is explaining where he has been and what he has been doing, he grabs a jug of wine by the neck. When the camera changes he now has it by the body of the jug. (00:07:30)
The Ghost of Harrenhal - S2-E5
Continuity mistake: Brienne and Lady Catelyn are with their horses by a stream talking. Brienne's white horse is walked into picture and is very dirty. The next shot the horse is clean. (00:38:40)
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 ★