He stopped dead in his tracks, thrown off by her extreme reaction. What was going on? Why was she so scared? Sure, he was a little freaked out by...whatever it was he had somehow just done, but the fact remained that he had helped her in doing so. And yet here she was backing away from him, looking at him as if he was some kind of murderer or something.
And then it hit him. That's exactly what she was doing. At first he thought she had been scared by his sudden display of powers (as he was), but he now saw an expression of recognition in her eyes. He had wondered before if she knew him...now it appeared that not only did she, but she was terrified of him. But why?
He looked nervously from side to side, making sure there was still no one around who would have heard her angry yell. The coast appeared to be clear, so he took a few tentative steps toward her again, raising his hands in a manner that he hoped looked as non-threatening as he intended it.
"Wait, please...you don't understand! I'm not sure what's going on here! I think..." He thought back to what Narcissa had told him, and why she had suggested he find this girl.