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.
Character mistake: Davos advises Grey Worm to claim land in the Reach and start his own house. Since Grey Worm is a eunuch, he cannot start his own house, as he cannot father children to pass his name and lands onto.
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 ★