I know I'm coming to this thread late, but here goes.
The first pic leaves me cold because there really doesn't seem to be much going on. Maybe if you had her leaning into her hand, head tilted and more expression it would work better. The pic as it though seems to lack a focus point and snap.
Her expression in the second photo is better but the combination of leaning back and the knife seem at odds. I think a more agressive pose, maybe on her knees attacking into the camera with the knife. Also, the picture as it stands her face appears to be slightly darker than the rest of her, I think you'd want to highlight the face and eyes.
I'd suggest also looking at different lighting to bring more snap into the image. Key light on the face and body, hairlight slightly behind and above and fill to bring out the countours or bring out highlights.