I would say he decided power is more important for him than love and if he will have a power over the person he wants, he will be in control. In the dungeon there was no Tav who said to him "Astarion, don't do this, this in not you, this is wrong" and he reassured himself that power gives him everything he wants. Which is not true, but at this point he won't change his mind.
Originally Posted by andromeda087
Well, where does it say that that is what happened? Please, direct me to the source, no speculation or assumptions, please.
Do you mean the writers video? (I'm not sure the question is for me or for Dotsat, so just in case) It's this one:
