Merlin and his apprentice take it all to hell, but her screams made him feel bad. He never hit girls and he'd been taught never to hit girls. The only boy/man in a housefull of women (his father's brief and infrequent stays never really counted), the thought had been emphasized his entire life.
He squeezed her wrist tightly enough to bruise and hopefully make her drop the blade, but not to break her wrist, though he could feel the bones grind in his grip.
"You're not a girl, you're a psychotic bitch. Go rot in the hell of my choice," he said, slapping a portkey he'd been given onto her and sent her reeling off to prison.