"You're…A Gur? And a monster hunter?" You can hear the disappointment, the disgust in Astarion's voice, how could he take so long to realize what you truly are?

He allowed you to get close, too close to comfort, more than he allowed anyone else, and now you betray him like this, he had no idea, he wouldn't have done half of the things he did for you if he knew his companion was a Gur all along. "I…I really don't know what to say." He mutters following with a long sigh, he's conflicted.