Continuity mistake: While Jaime and the High Sparrow are standing beside Myrcella's body in the Sept, in the overhead shot of Myrcella her right hand is placed over her left hand, but in other shots it's the opposite way. (00:18:30)
Continuity mistake: In one shot Sansa is writing a letter to Petyr Baelish, and is pressing a seal onto hot wax on the lower right corner. The next shot Sansa raises the letter, and the wax is gone. (00:39:45)
Battle of the Bastards - S6-E9
Continuity mistake: When Jon Snow is all alone in the field drawing his sword to fight the incoming enemy cavalry, behind him we see an empty field. but seconds later as the enemy cavalry is near him they are met by Jon's own cavalry from behind. (00:37:30 - 00:38:20)
Continuity mistake: When Sam hands the bureaucrat the letter from Jon Snow, in the shots facing the bureaucrat, the wax seal is on the left side of the envelope (when seen from Sam's perspective). In the shots facing Sam, the seal is on the right side of the envelope.
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 ★