So little is known about her. Makes me wonder if I should feel sorry for her going to hell. For all we know, she could be some psycho yandere who deserves it.
From what I understand, her boyfriend decided to break up with her. This scene comes just after he told her as much. As you can see, she didn't take the news very well.
My theory, and it's just a theory, is that she attacked him in a momentary fit of jealous rage -- probably not meaning to actually kill him, or even believing she could kill him. But once the moment was past and she saw that was precisely what she had done, her guilty conscience drove her to suicide out of remorse.
