Yeah, to get a more satisfying ending you have to
defy Bhaal after the Orin fight. It does not mean you have to suddenly be nice - in a dialogue choice afterwards you can say that you finally can kill for your own pleasure instead of Bhaal's. Which makes you a pretty horrible person and a good match with Ascendant Astarion. You still get the "you are the Heroes of Baldur's Gate" speech, wich is funny considering you are a villain at heart and might already plan to drench all of Faerűn in blood. But at least you can have a talk with your LI afterwards.
I agree that the pro-Bhaal ending is very unsatisfying - why do I have to take over my companions, especially my LI? There still could have been room for a talk afterwards with them, even if your are a super villain.
