Blackfish: Kingslayer.
Jaime: Blackfish.
Blackfish: I assume you're here to fulfill the vow you gave my niece. I don't see Sansa and Arya.
Jaime: I don't have them.
Blackfish: Pity. Do you wish to resume your captivity? [Jaime shakes his head "no."] Then why are you here?
Jaime: You know why I'm here. This castle belongs to House Frey. You're trespassing. In the name of King Tommen, I order you to surrender, or...
Blackfish: Or you'll kill Edmure. My nephew's marked for death no matter what. Hang him and be done with it.
Jaime: It won't stop with Edmure. You'll force me to storm the castle. Hundreds will die.
Blackfish: Hundreds of mine. Thousands of yours. If you can breach the walls.
Jaime: We'll breach them, and kill every last one of you. But if you surrender, I'll spare the lives of your men, on my honor.
Blackfish: Your "honor." Bargaining with oath-breakers is like building on quicksand.
Jaime: The war is over, Ser. Why sacrifice living men to a lost cause?
Blackfish: As long as I'm standing, the war is not over. This is my home. I was born in this castle, and I'm ready to die in it. So you can either attack, or try to starve us out. We have enough provisions for two years. Do you have two years, Kingslayer?
Jaime: You clearly have no intention of saving your men's lives. Why did you come treat with me?
Blackfish: Sieges are dull, and I wanted to see you in person, get the measure of you.
Jaime: Well now you have.
Blackfish: Aye, now I have. I'm disappointed.
Brother Ray: Violence is a disease. You don't cure a disease by spreading it to more people.
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 ★