Astarian is key to understanding the evil path. The parasite has freed him from his master, allowed him to walk in the sun, to enter houses uninvited and he wants to know if its possible to take the benefits and leave the rest behind. What if the plan isn't to make you mindflayer? If the absolute wants to him to become the strongest vampire lord in Faerun, well, great! So what if the masters are mind flayers? These masters seems less demanding than the last.
Not if I become the strongest vampire lord in Faerûn first.
And there are reasons to believe that the tadpole can be harnessed. The absolute wants the survivors of the crash dead. Could it be that the process was interrupted and the protagonists have more control over the "absolute powers" than the absolute intended? Perhaps there is an opportunity here -- infiltrate the new church, find the weaknesses and exploit them.
And you can just be seduced and tempted. I mean really early on you get a powerful set of gloves soon to be followed by a shield and amulet that can only be used if you take the brand and shout the absolute's name. Add to that powers from your seductive dream partner and the promise of drow bewbs and you've been seduced by the power of the dark side.
But then you are punished severely in almost comical ways for doing that. The evil path gives you less benefits than the good path. If the True Souls you meet keep wanting to kill you upon any mention of the tadpole or even if you get in their way, why would you play second fiddle to them? A pragmatic evil player realises Halsin is their best bet, but this makes them "good" when the help the Tieflings out of the interest of self-preservation.
