Dialogue with Nettie about removing the tadpole:

I persuade her to let us leave, and accept the wyvern poison, saying I'll use it if the time comes. Asatarion disapproves. After the conversation, he has an exclamation mark over his head. I talk to him and tell him that people will believe whatever they want. He cheers up, saying let's drop the poison down the nearest well and be on our way. The conversation concludes.

I feel like Astarion should have approved in the exclamation mark dialogue, canceling out his initial disapproval. That's how those lines of dialogue play out.