@Add1152000
They went to help the elf on the assumption that she and her contracted spirit knows about the existence of their village.
They killed the mercenaries and the slave trader because he told them about the existence of the village himself. The reason why he told them was so he could lure the mercenaries deeper into the forest to prevent any of them from escaping. On the other hand, there was no reason to kill the elf and the draph. I think that if William only rescued the captives, the slave trader might go deeper into the forest to retrieve them, so there was only really one solution.
William is helping Cornelia out of empathy. He probably doesn't care about the elf though. Cornelia's wishes is tied to Meldina's, so if the elf wants to leave, then the draph will leave too.