FINISHED! I imagined for this the idea that Astarion is fully manipulating and seducing Wyll for his own purposes (killing Cazador), and Wyll is well aware of that. What changes is that Astarion feels like being close to Wyll is dragging him away from his need to conquer Cazador's power and "tricking" him into falling in love and caring for him (someone who is under the power of his warlock contract, and thus does not have free will yet).
Astarion is trying very hard to ignore the fact that the dumb idiot he is falling in love with is about to receive the Volobotomy Special right next to him.