Continuity mistake: When Ser Barriston throws his armor and sword on the floor after being demoted, the sword stops by his armor. However when Sansa kneels down, the sword is now a good distance from the armor. (00:52:40)
Continuity mistake: When Catelyn meets up with Robb's forces, she hugs her son and then puts her hand on his shoulder/chest. In the next shot, her hand is down by her side. (00:41:40)
Continuity mistake: Arya and Syrio are practising at the start, we see them stop with their swords touching and crossed. Camera changes and Syrio's sword is much lower and no longer touching Arya's. (00:02:10)
Revealing mistake: Just after Jon is told he is confined to quarters, there is a shot of outside with snow falling. However the snow is actually rising. Indicating the shot has been reversed. (00:22:55)
Continuity mistake: Catelyn confronts her sister about the raven's message that arrived at dawn. When she does, the roll of paper is rolled completely differently between the 2 camera angles. (00:16:05)
Continuity mistake: When Ser Barriston throws his armour and sword on the floor after being demoted, the armor and sword are inside the black tiled square on the floor. Camera changes to show Sansa kneeling before the council and now the armor and sword are outside of the black tiled box despite no-one touching it. (00:52:30)
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 ★