Other mistake: As the surviving characters discuss the fate of Westeros, 2 plastic water bottles can be seen tucked behind the chairs they're sitting on.
Visible crew/equipment: When Bran joins Tyrion and the others at the small council meeting, Bran begins by saying that they appear to be missing a Master of Whisperers, then just as he says, "And a Master of Laws," the boom mic can be seen dipping into view over Bran, at the top of the screen (visible on Hulu). (01:11:00)
Continuity mistake: When Daenerys dies, in the close up her head is parallel to the ground, looking up at Jon. Cut to a wide shot and her head's hanging much further backwards, plus her eyes are suddenly closed.
Continuity mistake: When Dany dies, the blood trails coming from her mouth and nose are pale in some shots and dark in others. They also appear to change position slightly, but that might just be the camera angle.
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 ★