Nightwing wasn't even going to bother arguing with X about whether or not he was dangerous -- he'd killed at least two people, that made him dangerous -- and instead focused on the rooming suggestion. "No. You're not getting a roommate. He can come visit you if he really wants, but he's not staying in here with you."
Batman was probably watching them on the cameras, he realized, and Nightwing was probably going to get hell for what he was about to do, but there really didn't seem to be much threat of the two trying to get out. Besides, for whatever reason, X seemed to like Cake. "I'll be right back, okay? I'm just gonna go grab my lunch from my room." He locked the door behind him when he left and hurried back to his room, not gone more than two minutes before he'd come back and perched himself on the table, offering Cake half of his sandwich.