It's an interesting discussion. I've never played as Astarion but having played the game many times as other OC's and having had Astarion in my party many times it's not too difficult to start running through the various interactions we are all so familiar with.
I think the idea that it's difficult to imagine Astarion choosing a route other than tadpole-kill-ascend is valid. After all, he's an undead vampire, so default-wise he's more or less already on the evil side of the ledger from the gitgo. His character also has a predisposition to embrace the tadpole as he admits in ACT I. The temptation of Ascencion once he learns of it is also pretty hard to resist.
His personal story/quest is a fugitive on the run, so he's motivated by fear as opposed to some OC's who are either mission goal-directed (SH, Wyll, Lzl) or already engaged in a potential redemption arc (Gale Wyll). Curiously Kerlach is also a fugitive, though she exhibits a "just cause" motivation of revenge. I think like Astarion it's hard to play Karlach logically any way other than her obvious "good" character.
Perhaps its simply a case of "yeah Astarion's evil", there is a possibility to pursue a redemption type story but it doesn't make a lot of sense. Likewise I think Karlach is "good" and while it's possible for her to turn out evil (AFAIK) it doesn't make a lot of sense either.
I think its good that not every OC is a dual personality "twoface". I never take Astarion in a good party, nor do I ever take Karlach in an evil one. In fact there are some hard limits when taking Karlach and Wyll as followers that keep you from pursuing some evil paths which would suggest that there are some actions by them as evil avatars that should not be possible.
