Then you might have gotten a wrong impression.
The lore is full of themes of fall and! redemption.
Not just falling. (Altar of the Luonnotar, The Elhoim, Decius chosing to go to the Malakim for refugee, the empyrean. The tale of Saverous, Brigit, the positive outcome of the radiant guard just to name a few of the themes that actually are more about redemption.)
Look up Sheelba's and Rantine's entry in the pedia for clarification that evil is not necessarily evil to the core (at least in an ethical sense).
Evil in FFH always means that one actively or passivly helps the cause of the gods who oppose the one (so are in Agares league of Gods wanting to destroy creation or mostly. Mulcarn excluded to a certain extent....). Thats the very definition of it in terms of FFH2. Good is opposing the evil gods. Neutral is taking no active part in the conflict.
Sure for most evil leaders it seems more clearly cut (far from all of them though and few are completely beyond resonable chances to go down a different path. Hell (
), even the Calabim might turn to the Order when the World turns to hell and they starve because their "lifestock" dies out. Still surely Calabim lands whould not be a nice place to be though...
).
But among good and neutral ones its very much blurred by our standarts (with some of the "good" leaders in FFH actually being quite malicious.).
The context of erebus of course is an utterly different one. Thus its inhabitants might judge Order Mercurians under Basium very much as a Savior for example.
Doesn't mean it doesn't turn out evil in many circumstances of course (in the Lore at least). But redemption is not impossible.
So yes its quite picturable that a Sheelba returning to the ways of Junil (Bigit might be involed here redeeming Bhaal or possibly taking her place) or a Mahla throwing her weight behind Kilmorph and allying with the dwarves for the good of her people actually directly or indirectly oposing Agares and his gang in the end.