Lust rewarded him more more pleasure as she 'attempted' to wriggle herself free. She didn't tell him to stop or make it seem as though she was in true pain, because he wasn't ready for that yet. If she brought forth tears or a desperate plea his own morals would managed to snap him out of this.
"Would you still get a rush out of murder?" Lust asked him, smirking, spreading her legs a little in the guise of trying to free herself. "We could bring another kid up here for you to kill, if you like?"