Bullying is a weird thing. When people fight back against it, it is surprisingly common for the response to be completely out of proportion - partly because a long period of anger and resentment is fuelling it, so that the response is motivated by pure rage, rather than assertiveness. When people are that angry, they often lose control.
There is a moral to this story, which is if you bully someone relentlessly, at some point you may come off worse.
All that said, while my sympathy for the girls may be slightly limited, I do have sympathy for them because their crime is in no way proportionate to the punishment they've received, and it's right that the bullying victims' experiences at the hands of these girls was only allowed to mitigate his sentencing to a finite extent.
The sad fact is that society would go to hell in a hand cart very quickly if being ill treated became justification for murder and extreme violence. Far better to use the available laws to stop the mistreatment before things reach that point.
That in itself of course raises wider concerns - why don't we take bullying in schools and the workplace more seriously? Why don't we do more to deal with relationship bullying?