I have high hopes for the evil path, and I think it could be really cool. Right now its more stupid evil though, since joining the evil guys rids you of the best chance of healing you believe you have at that point. There need to be better incentives, and most of all, they need to be better communicated. Also, I wish for once in a game the evil path wouldn't have you mindlessly murder everybody.
How is Halsin 'the best chance of healing you believe you have at that point'? Even before you meet him his lab proves that he only met people with the 'improved' tadpoles recently and likely has no clue how to remove them. Infiltrating the cult is obviously the best chance considering that they actually know how the tadpoles works or can point you to the tadpole's origin, so earning trust with the cult by (at the very least) not murdering True Souls you meet makes much more sense for a rational selfish person. The second best chance is githyanki creche, because they've been studying illithids for a long time and may have some relevant information. Halsin seems a weak lead, which is proven later by him being capable only to point you towards the cult HQ, something you can guess on your own.