Like I said, I don't think the change had anything to do with Halsin. I think they decided to change Isobel's place in the wholse plot and the timeline of things with Kethric and when they did, it didn't make sense for Halsin to have killed her. Kethric turned to Shar because Isobel died, and Halsin only fought Kethric because Kethric turned to Shar, so she had to already have been dead before any first battle where Halsin could have killed her.
I don't really think him having killed Isobel would change much anyway. Even if that had happened, he would still be absolved of those issues after the curse ended, and him continuing to travel with us would still be because he's grateful for our help, which is his motivation currently. Guild over killing Isobel is just an extra tie to the plot that's maybe nice to have, but doesn't actually change anything.
