He sees their throat work as they swallow, their jaw clench and unclench. They have trouble speaking when they're upset, he knows. He just waits, hoping for one answer but knowing the second before her mouth opens that it'll be the other.
"No."
"Taran." He goes to her. She steps away. Her withdrawal sends a knife through his heart but he doesn't pursue, just stops where he is. "Taran, think of it. He will never hurt me again. No one will ever hurt me again. You - you'll want for nothing. And - " He feels his throat close over; he swallows around it but his next words come out choked regardless. "And I can walk in the sun with you."
"Astarion." His name is a whisper that somehow still echoes through the chamber. "Fuck the sun. I will walk in the shadows with you every day for the rest of my life. Happily. As long as you're you." They jab a finger towards Cazador, still prone on the floor. "Not him. Because if you do this, you will be him. Worse, because you had the chance to be better and you threw it away. No." She shakes her head. "No, I will not help you. I will not do this. And if you do this, I will leave. I will not be a part of whatever you become."
Astarion looks back at Cazador. Turns back to Taran. He can feel the moment slipping away, all the convictions he held so strongly for so long draining through his fingers. That future he craved for so long, that power and freedom and authority, all turned to ash in his mouth at the thought of being alone through it all.
He drops his gaze to the floor as he draws his knife. "You're right," he says quietly. "I can be better." A brief flicker of a smile towards Taran. “You made me better. Whether I wanted it or not.” He turns back towards Cazador, flipping the blade over in his hand. "But I'm not above enjoying this."
And then it's over. Everything is...over. Astarion is on his knees on the ground and Taran is right beside him, arms wrapped around him, holding him so tightly he thinks he would break if he didn't already feel shattered.
"Thank you," they whisper. "Thank you. I'm so proud of you. I'm so sorry."
Astarion doesn't know what to say, to any of it. So he doesn't speak, just curls into her arms. And she just holds him through it all.