From War Crimes, Chapter 34:
Anduin whirled, sickened as his heart contracted at yet another betrayal on this dismal day. "Two females with crossbows," he said, bitterly. "One orc, one human, weren't they, Li? I should have known."
"Perhaps you should have, but it is not yet within you to suspect treachery, Anduin Wrynn," said Wrathion with a sad smile. If it is any consolation at all, I am deeply sorry for what I now must do.
Anduin laughed scornfully. "Sure you are."
The Black Prince shrugged. "Believe what you will, but it is the truth. We are friends, you and I."
"Friends? Friends don't kill each other!"
The dragon's glowing eyes widened and he looked almost hurt. "Why would I do that? Look at the Chu brothers. They are alive, though admittedly with rather terrible headaches, and I care far less about them than I do you.
"Wrathion, what is going on here? What are you doing?"
The young black dragon sighed. "You once asked me to watch and listen, and to make up my mind as to what is best for Azeroth. I have done exactly as you bid. You are the heir to the throne of Stormwind. You have a duty - to keep your kingdom safe. You do what you believe is best for it and its people. As the last black dragon, the former charge of my flight - to keep Azeroth safe - falls solely to me. I must honor that charge."
"Don't listen to him, Anduin!" cried Chromie.
Anduin gestured towards the still-chained pandaren. "This is keeping Azeroth safe?"
"In this case, I assure you, the end does justify the means. It is my deep hope that one day you will understand. And on that day, you and I will face a terrible enemy. Perhaps even do so as brothers."
Desperately, Anduin reached out his hand. "You don't have to do it this way. Tell me what's going on. We can work together. We can find some day to-"
"Farewell for now, young prince," said Wrathion. He lifted a hand, and Anduin knew no more.