I'm not sure what chapter but what's her face said that half elves fall into darkness most of the time so that could be the reason he is became like this just like when her uncle was denied marriage he fell into darkness and Niro got kicked out and then got beat and was was gonna kill that slave owner that showed that he was slowly falling into darkness then now he is even more mad that his dad is dead. I'm not sure it's only a guess :(
Thanks for the update