Continuity mistake: When Tyrion wakes up inside the skycell you can see two skylights in the ceiling of the cell through which the sunlight passes. However, when we get a long shot of the skycells, you can see there's no way for the light to pass through them, as there are no "holes" on the floor of the cells above Tyrion's. (00:13:45)
Continuity mistake: When Ser Vardis has cornered Bron right at the edge of the moon door the position of Bron's body changes between shots. When the shot is taken from inside the moondoor, Bron's body leans backwards, over the moondoor. But when the shot is taken from behind the throne Bron is leaning forward.
Continuity mistake: When the guard is hitting Tyrion in the sky cell, Tyrion has his arms outstretched in one shot and against the wall in the next. (00:14:00)
Continuity mistake: When Bran is taken from his horse in the woods, there are 3 wildlings that attack. The first Rob cuts his throat, the second, Rob is holding the female wildling by the hair, he then spears and kills the third with his sword and the very next frame the 3rd wildling is then holding a knife to Bran's throat, unharmed. (00:11:15)
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 ★