Rejecting Bhaal or rebelling against him and become a follower again later. Why should this not be possible temporarily? You can change your faith or worship several gods. Derived from this, you can always turn back to one god. Sarevok also became a champion of Bhaal again, even though he was only an ordinary human after losing his divine spark. How else could he still be alive so many decades after BG I?
Sarevok was brought back from death by Bhaal. In the murder tribunal, you can find a piece of a journal, where Sarevok regrets that Bhaal brought him back from the peacefulness of death, having to relearn all basic corporeal things, like breathing, again.
This brings me to another question. I read that Bhaal came back from banishment after the last Bhaalspawn died. (Absorbing the essence of the spawn and becoming whole again, or something like that.) After this, he apparently brought Sarevok back from the dead who then produced new spawn Helena and Orin. But where did PC Durge come from ? Did I miss some kind of hint in the game to explain how this new spawn came to be after the entire line died out ?
As for why I think Durge could become a monster, this idea probably came just from viewing the ending scene. The PC that you've grown to like (even as Durge) during all these hours of play, now has become a terrible being. Eyes wide open, never blinking, mouth chattering, and head jerking uncontrallably. You should see this, if you haven't yet. But even if you'd watch it in a clip from someone else, it would not have the same effect as it would not be YOUR personal PC that you see transformed like that.
Finally, for the banishment of the dead 3. It is my impression that one should take statements from Withers literally. He doesn't strike me as someone who uses figures of speech. So, when he says "thou shalt bother us no more". I take this as a judgement, meaning that the 3 will be gone and lost forever from the realms. (Or at least from any material plain where mortal souls reside) Also, he seems to speak for "the Gods" in plural, so maybe they filed filed a complaint against the 3 soul destroyers with AO and received permission to get rid of them for good.
