You left out the main reason for the attack. The thing that pushed the bullied girl over the edge was being raped by the girl who gets killed with a test tube from the science lab. The bullier and her gang hold the other girl down and then do her with the tube, all the while reminding her "if you move too much the tube might shatter inside you - and then what would you do?" They seriously fucked up her head. Also, bully girl was fucking her brother. The manga also has lots of incest, I notice no one mentioned it yet.
It's a VERY VERY dark series.
The anime is total shit, mostly becuase its only 12 episodes and doesn't cover the entire storyline so nothing is explained by the time it ends.