Do you think someone in good mental health would stab four people in a shop without apparent ulterior motive - e.g. robbing the place? If it turns out that it was pre-meditated then that's a different story.
As for the victim/villain angle, I don't excuse the act in any way, but if (big if) her mental health issues had not been addressed by the correct authorities then it just adds to the tragedy of it and one would wonder whether it could have been prevented.