Remember the core problem with the entire "evil" route is that the cult is a group of enslaved people that simply don't realize it, not to mention the fact that its a kill everything path with little to no reward. Your character over time learns that all these bad guys have tadpoles in their heads and don't know it. If these people knew about them they would most certainly not be serving the Absolute and instead committing suicide or trying to figure out how to remove them. That means the kick off to going evil is "Hey, these guys are all Mind Flayer slaves, surely nothing can go wrong if I side with them". The potential power offered is also pretty unconvincing, yes people make pacts with demons and other worldly beings, however there are several factors that motivate these choices some of which are:
1)Unsound mine
2)Overriding lust for power
3)Desperation
4)Trickery
5)The deal comes from a god or major entity
6)Other things I can't think of atm
These can be combined as well but the result is usually the person making the pact and then trying to figure out a way to get out of the deal while maintaining the power you've gained. The tadpole in your brain doesn't offer you anything grand and the costs are extreme as it seems that once matured you aren't simply killed but have your soul obliterated as well. I mean hell we didn't make some pact with a fiend, god or other world entity, we had a tadpole stuck in our head against our will. We also have very little indication that the Absolute is anything other than a Mind Flayer grand plan. Evil or not you don't live long siding with an extremely xenophobic alien race, it can never be stressed enough that Mind Flayers don't look at others as anything more than cattle. Also we (the character) are unsure why our tadpoles are different, big question in the story, where the other tadpoles seem to be functioning as intended. Maybe it's due to outside influence, maybe its due to Mind Flayer experimentation, maybe the process of insertion wasn't completed or was damaged during the attack all we know is that it is really bad to have it in your head.
Sure someone like Astarion gets some pretty sweet benefits, they're temporary and end in a bad day. Maybe he'd be willing to risk simply controlling the tadpole due to the benefits but if anything were to go wrong at anytime he dies brutally. Personally having clear and concise lines for good and evil rarely works well in games as the result is typically genocidal or mustache twirling evil and practical/intelligent good. I'd say it would be better to have a single path with various decisions and branch outs rather than two seperate paths with one evil and one good. Though for this story we have no idea what Act 2 or 3 entail.
