Cripples, Bastards, and Broken Things - S1-E4
Daenerys Targaryen: If you ever lay a hand on me again, it'll be the last time you have hands.
Tyrion Lannister: If the day ever comes when you're tempted to sell me out, remember this: Whatever their price, I'll beat it. I like living.
Cersei Lannister: When you play the game of thrones you win or you die. There is no middle ground.
Tyrion Lannister: The Eyrie. They say it's impregnable.
Bronn: Give me 10 good men and some climbing spikes... I'll impregnate the bitch.
Arya Stark: They said you found the bastard and the wolves are fighting the lions and the savage... Something about the savage.
Ned Stark: Where did you hear this?
Arya Stark: In the dungeons, near the dragon skulls.
Ned Stark: What were you doing in the dungeons?
Arya Stark: Chasing a cat.
Robb Stark: Good. Next time I see you, you'll be all in black.
Jon Snow: It was always my color.
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 ★