The point was there was motive and intent by the defendants to "humiliate" the girl and one of their last messages sent was in essence a suicide request, and unfortunately the young girl was messed up in the head enough to take it to heart.
If there's enough evidence to show that there was malicious intent, I feel the mother and daughter that sent these messages should take responsibility for indirectly contributing to the turn of events led by this young girl. Sadly in this case, she took her own life.
Unfortunately personal responsibility for one's actions doesn't exist in this day and age, especially not in the court room. It's all about passing blame.