Responses
I think it's more like
"If I can't have him, at least she can’t have him either."
Connected with (a little bit of) delusional hope that she might have chance with him then. Her EQ is quite low since she has been spoiled rotten since childhood and there probably hasn't been anything she couldn't get as long as she wanted it. It kind of feels like she thinks of him like a thing she can just possess, not like a person with his own will and feelings.
I don't really think it makes the storyline crazy - it's just a crazy chara^^

I'll never understand why rejection is so hard to grasp for some people / characters