It was so strange, for Terrence, to feel proud and gratified and awful all at once. He honestly couldn't remember when he'd liked Katie more, or felt more certain that they were meant to be friends, and yet in this specific circumstance, he'd also alienated himself from her, made himself the enemy. Fucking hell.
"I didn't know," he said, eyes narrowing. It was justified that she should accuse him like this, but he had the right to defend himself, too. "I hadn't taken the Mark when I'd first met you, I didn't have it when they took your mother, or even when she died."
If she thought about it, maybe she'd realize that he'd gotten the information about her mother, and pushed her out of the control of the potion, at the risk of both his own life and their friendship. Although, he wasn't terribly surprised that all he was getting from her at the moment was distrust. It wasn't as though he hadn't earned it. If she was going to hate him, though, he'd rather it was for the right reasons, because she knew the truth and couldn't come to terms with the things he had done.
"Also, I suggest you keep your voice down," he continued, calmly, though his eyes were flashing. "I'd hate to have to take any steps to get rid of any witnesses, for both of our sakes."
By which he really meant, he would probably use a memory charm; he wasn't nearly as good as Ella at affecting other people's minds (and not as good as he'd like at protecting his own mind anymore, either), but he was fairly sure he could manage it. But leaving it ambiguous made the threat seem more ominous, and he really did need her to calm down immediately.