I think the story just changed. Based on what we have now, it wouldn't make any sense for Halsin to kill Isobel. They're both good aligned people with no reason to clash at all based on the story presented. I just think Halsin is another example of Larian being flakey and irresponsible in their creation process. Poor planning and poor consideration.
There was sense and it still would be there if Larian hadn't changed Nightsong: originally she was... something like a Herald of Shar or something similar and she was a reason for this event. There was logic in her name since Shar is a Night Singer. Calling daughter of Selune "Nightsong" is weird, plus it is weird she is a mere aasimar (correct me if I am wrong but aasimars are basically the same thing as tieflings, but with angelic ancestry). She should've been aasimon/deva/angelic being, but that's a discussion for a different thread.
