Kael'thas: Kel'Thuzad has been destroyed. No! I should have been the one to claim vengeance for the sin'dorei!
Accuser: And what would that have accomplished? Would it have changed the fate of your people?
Kael'thas: No... I merely--
Accuser: So this retribution would not have been for them at all. Only for you.
Kael'thas: I... I was not there when Silvermoon fell. Absent in its hour of greatest need. My failure has haunted me ever since.
Accuser: Then it is not vengeance you seek, but redemption.
Accuser: You committed many sins in life, Kael'thas Sunstrider. Your kingdom's ruin was not one of them.
Accuser: Repent for all else... but forgive yourself for that.
Kael'thas: Perhaps... you are right, Accuser. It seems I have work to do.
From the book Arthas: Rise of the Lich King, pgs. 248-249:
"Your petty magics will not stop me, Antonidas. Perhaps you've heard what happened in Quel'Thalas? They thought themselves invulnerable as well."
Jaina thought she might be sick. She had been in Dalaran when word had come, from a handful of survivors who had managed to escape, about what had happened to Quel'Thalas. So too had been the quel'dorei prince. She had never seen Kael'thas so--so angry, so shattered, so raw. She had gone to him, words of compassion and comfort on her lips, but he whirled and gazed at her with such a look of fury that she instinctively drew back.
"Say nothing," Kael had snarled. His fists clenched; she could see, to her shock, that he was barely restraining himself from physically harming her. "Foolish girl. This is the monster you would take to your bed?"
Jaina blinked, stunned at the crudeness of the words coming from one so cultured. "I--"
But he was not interested in hearing anything she had to say. "Arthas is a butcher! He has slaughtered thousands of innocent people! There is so much blood on his hands that a whole ocean could never wash them clean. And you loved him? Chose him over me?"
His voice, normally so mellifluous and controlled, cracked on the last word. Jaina felt quick tears come to her eyes as she suddenly understood. He felt helpless, impotent, and was striking out at the nearest target-at her, Jaina Proudmoore, whose love he had wanted and failed to win.
"Oh... Kael'thas," she said softly, "he has done... terrible things," she began. "What your people have suffered--"
"You know nothing of suffering!" he cried. "You are a child, with a child's mind and a child's heart. A heart that you would give to that--that--he slaughtered them, Jaina. And he raised their corpses!"
Jaina stared at him mutely, his words having no sting now that she knew the reason for them. "He murdered my father, Jaina, just as his murdered his own. I-I should have been there."
"To die with him? With the rest of your people? What good would throwing your life away do for--"
No sooner had the words left her lips than she realized it was the wrong thing to say. Kael'thas tensed and cut her off sharply.
"I could have stopped him. I should have." He straightened, and coldness suddenly chased away the fire in him. He bowed low, exaggeratedly. "I will be departing Dalaran as soon as possible. There is nothing for me here." Jaina winced at the emptiness, the resignation in his voice. "I was a fool of the greatest order to ever think any of you humans could aid me. I will leave this place of doddering old magi and ambitious young ones. None of you can help. My people need me to lead now that my father--"
He fell silent and swallowed hard. "I must go to them. To what pathetically few remain. To those who have endured, rebirthed by the blood of those who now serve your beloved."