Why Did I Turn the Qing Leng Sword Immortal into a Yandere?

My name is Gu Ping'an. I was picked up and raised by my sister when I was 7 years old. Many years after my sister left, I met someone who looked very much like her. I took her as my apprentice, and my companionship and care day and night actually planted a forbidden fruit in the girl's heart. When I realized that the way she looked at me was wrong, it was already too late... "Master, you can't run away!" "Come on, Master~, it's okay even if you think of me as that woman~"