I've seen friends in abusive relationships and know how they can quickly escalate and get out-of-hand. I think this poor girl truly feared for herself, at the point of escalation...he is described as a pretty big guy, after all. Seeing as she had previous abusive marks, it's clear that this was not a "first time" incident. It also sems that her Guardian Angel led her hand...actually delivering a killing-blow to the torso is a pretty rare thing, unless you happen to be someone whom would "know", and I doubt she's a Pro-Killer.