Well, this one is not the yandere I want and expect. It seems that she already had a partner before but was disappointed with him because she specifically said that she wanted MC to be her first partner. But then somehow remembered that some child had promised to marry her back then then coerced him. I don't know, I feel that if she remembers and clings to that promise from back then, then she wouldn't get another partner (well, MC certainly had another partner, but he didn't cling to that promise, who was the yandere here?). Now that she failed her relationship, apparently saw MC being happy with his partner, remembered the promise, then coerced him out of desperation.
So in conclusion, this one is bad for me.