Even yet, the most common result would be for the rejected one to go off moping, maybe even to a different school. How often does this result in murder? I bet there were other “cries for help” that the supposedly sensitive set simply missed.
Oh no, she did the worst thing she could do to him: Humiliate him in front of his peers.
He did not deserve that and he took that as a form of social death in his limited world (the school). He could never walk the hallways of that school again without the stares of ridicule and howls of laughter at him. He was finished there. Should he have left her alone and sucked it up? Yes, of course that instead of killing ten people but his emotional reaction to her public humiliation of him was more than he could bear psychologically.
She did not deserve to die, for any reason.