Iron Woobie: Konata, ironically enough, qualifies. After all, she's been trying so hard to tell Kagami her true feelings towards her, but she hasn't succeeded for more than 3 years. And then she witnessed Kagami suffer the mother of all beatdowns at the hands of Sakura, after having a nightmare about it the previous night. But what does she do? She starts fighting back. Hard. Because of this (and the fact that she was also injured during the fight), she becomes a good example of this.
Another possible example of Sakura crossing the line is that when Kagami starts to cry after begging her to stop, Sakura responds by laughing in her face, slashing her with the knife again, and to top it off boasting that nobody can save her now.
Nightmare Fuel: Pretty much everything about Sakura, but the worst thing about her is how disturbingly realistic she is. That brutal No-Holds-Barred Beatdown that she inflicted on Kagami? That kind of stuff actually does happen sometimes in real life.
What an Idiot: Sakura really should have known better than to try to get even for something that wasn't worth getting even over. And at her trial she doesn't even attempt to defend herself, instead insisting that Kagami somehow had it coming.
The Woobie: Kagami forobviousreasons, during Chapter 4 after Sakura inflicts the mother of all beatdowns on her. Seeing her sprawled out on the ground and crying her eyes out from the pain of her injuries, it's hard not to feel sorry for the poor girl, especially if you already thought she was a Woobie in the main series.