It was obvious that Enigma was willing, to some extent, to be a party to whatever Crane was up to. That didn't mean that Dinah was okay with him being violent with her. The grin was unsettling, but in Dinah's eyes, she still seemed like a child who might have a chance. Either way, she reacted quickly and decisively. She crossed the space between herself and Jonathan easily and threw a punch.
Some might argue that she was quick to violence and maybe in these sorts of situations, she was. It was also compounded by the fact that this place had touched a nerve, particularly regarding violence to children.
She might have seen Jake's face for just a moment. Jake, pushed to murder by the Joker's torture. Jake, bleeding and beaten and broken.