Too many people move too quickly from a fact - a girl stabbed 8 people with a knife - to looking for a theory which might mitigate or even justify these actions. Ironically, if the crime is rape, we're told its wrong to "blame the victim", but it seems that some are pretty quick to "exonerate the perpetrator" in crimes involving teenagers (which is really just a variant, especially in this case, of blaming the victim).
The flaw in the approach that social engineers are taking in relation to bullying is that they are completely focussed on one side of the equation. The "problem" is attributed 100% to the person who is teasing, harassing, insulting, or doing anything else that is currently defined as "bullying". There needs to be some focus on why some kids are not able to cope with even a normal amount of teasing from their friends and peers. Some of these kids are ticking time bombs. Even if they manage to get through school in a bubble of protection, nothing will insulate them from criticism, teasing and insults that they WILL encounter in the real world. People need the skills to cope with this inevitable aspect of life.
However, responding to teasing and harassment with physical violence is never an acceptable way of coping. You cannot respond to verbal harassment by escalating matters to a physical confrontation. That might have been what our fathers taught us back in the day, but today that leads to criminal charges and civil lawsuits. We've got to stop looking for excuses for violence. It's sending the wrong message to kids.