I'm definitely expecting some kind of dramatic reveal once we reach Baldur's Gate and (I assume) confront Cazador - something beyond finding out what, exactly, is on Astarion's back. And you're right, that's probably going to be the point at which Astarion does or doesn't (for me) transition from "entertainingly despicable" to "just plain despicable". Right now Larian's doing a good job of toeing that line with him.
