In my adventure, I killed them, although I didn't really want to do it. They didn't seem evil, either by dialogue or by appearance. I just needed a fighter in my group, because I play for the Priest myself. Apparently it is possible to leave them alive, but I did not abuse save/load and on my first attempt I did not see an opportunity to release Lae'zeland not kill the Tieflings. And yes, I also feel sad about it.
Maybe just need to make more hints in the dialogue to keep them alive and at the same time to free Lae'zel?
