@MaxBedlam
Yeah, looking through the pages again, I think what happened is that Satsuki just knocked her out, then cut herself and spread the blood around to make it look worse. The bloodstains on her hand look like they would come from a cut on the hand itself rather than a Extertal source too.
That way she would both prevent her from actually dying (due to not being were the premonition said she would die at the right time), and possibly scaring her enough to stop the bullying.
Her just suddenly turning into a murderer doesn't really make sense, and we already know she stopped caring about her own reputation anyway.