I think a dad who can teach his daughter what is right and wrong is a whole difference between Som and Annusaniya and the main reason why Som wanes her route to evil. Som does not have someone to teach or guide her. That's why I said Som is not prone to evilness but her surrounding shapes her for who she is. When Wat scolded Som with reasons, do you see her not being able to talk back but silent for a while recalling her action? That's why Som needs Wat so badly.
But I'm sorry I'm in defence of Nuth as well. She has a trust issue cuz of her aunts. So the only two people who won't backstab her as she believes are her uncle and dad. So she sticks to her uncle like glue becuz she is not sure if she will have any person like Chaya who is loyal and also there for her. Wat must crack this wall that Annusaniya builds. He has to show he cares for her without any condition. His love must be unconditional to reach Annusaniya's heart. He is slowly getting there. Do you see Annusaniya smiling a bit when Wat takes care of her burn? She is gradually recognizing Wat has no evilness in his heart. That's actually Wat's very first success knocking at Annusaniya's heart!