Continuity mistake: When the Faith Militant descend on Ser Loras Tyrell's training session there is a shot after the Militants have come down the stair of Loras fighting from behind a man's shoulder, in this shot you can see they are fighting on grass surrounded by walls with sheer drops with no entry point other than the stairs the militants walk down. In the next shot you can see the 9 militants that have come down the stairs approach Loras from one side, but suddenly 6 militants join him from the opposite side, where they definitely were not before. In the zoomed out shot you can see there is nowhere for these men to have come from. (00:11:15 - 00:11:50)
Continuity mistake: Towards the end there is a body of a dead Son of the Harpy lying between the fallen Unsullied and the side corridor visible in the wall, located very close to either. In the next show the body is gone. Also, as the camera angle changes, the blood stains on the walls look differently. (00:48:50)
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 ★