It's become an obsession

My fiancé thinks that he's a Slytherin when he clearly isn't. All his friends and family have told him, but he just won't listen! At first, it wasn't too bad, but now he's started acting cruel and malicious in order to seem more Slytherin! It's tearing his family apart, putting a barrier in his friendship circle, and wrecking our relationship. He's not the man I met. He's becoming obsessed. How do I persuade him he isn't a Slytherin? Thanks!

Hi, Sounds like he’s got a low opinion of himself or somethings happen to make him spiral. Counselling might be the best step to help him.