And now we see the source of future Sauro's single minded focus. He's trying to fulfil the promise he made, thinking that he has to because his friend asked him to do so.
Also appears like Sam was trying to inhabit Arsenyo, likely because of the seal drawing him in, but luckily Cait Sith made him flee. For now.
As for that twin, yeah it seems very possible for her to want revenge. She lost her mother, and in her mind it was Arsenyo who killed her (since even though she was running from here, she didn't want her to die as seen when she and her sister tried to stop Sauro and almost got him killed), and now because Sauro only cared about killing the witch for what she did to his friend, her sister died too. Plus her father is also dead from the look of it. She's lost a lot of people in a short space of time.
Very easy to see her turning to witchcraft as a result.