Thanks for the confirmation!
I agree Elgohar - actually, unless our character is either chaotic evil crazy, falls madly in love with Minthara at first sight, suddenly doesn't want to remove the parasite, or thinks the only way to actually remove the parasite is to play along with the Absolute true souls, the evil path of siding with Minthara really doesn't make much sense. None of those circumstances are expressed in dialogue, journal, cutscene, internal thought...
It'd make much more sense, even if evil, to rescue Halsin so he can help us - potentially killing him after he cures us, whatever. But siding with Minthara now either means we, quite surprisingly, choose to keep the tadpole, think for some reason no one can cure us except the source of the Absolute, or are just solidly enchanted by Minthara and will do whatever she says.
I hope the narrative leading to this evil path is developed a bit more to make the choice more sensible =)