"Wait! What do you think you're doing?"
Robin resented the black mage and his subordinates for what they did but she couldn't tolerate Arthur's actions. She couldn't bear to watch him brutally behead a little boy.
Her reasoning was obviously wrong as the boy may be just young in appearance, he may even be a monster or some kind of shape-shifter. Having a weak heart in such times can be quite troublesome. If you spare someone because of pity or sympathy, there's bound to be terrible consequences, no matter how insignificant the person you spared was.
"I'm getting rid of an enemy. Stop pitying enemies and try to remember what this crazy cult did to your grandfather. I'm not trying to be cruel to you but it's because you let appearances trick you or you spare anyone that looks weak is exactly leads to consequences you are not ready to face."