Continuity mistake: When Ser Jorah tells Dany that her life with Khal Drogo "will get easier," watch the strap of her dress. It moves up and down her right shoulder throughout the scene. (00:03:05)
Continuity mistake: When Tyrion is talking to Sandor, his position in relation to the gate changes between camera cuts. In the first shot he is next to the gate. In the second shot he is about 3ft away. (00:06:20)
Audio problem: When Tyrion is saying he wants to stand on top of the wall, it is an over the shoulder shot, however his mouth doesn't match up with what he is saying. (00:07:40)
Revealing mistake: When we see Joffrey get attacked by the dire-wolf, You can see his arm suddenly has padding underneath his jacket. (00:47:20)
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 ★