Regulus' arm shook from all the effort he was putting forth into keeping his wand level and ready for action. It was true, he didn't actually recognized Castiel, so if he were in his right mind, he would definitely wonder how this stranger could possibly know what he had done. How could a stranger turn him in? Unfortunately, Regulus was not in his right mind, so this thought never occurred to him.
"He's here!" Regulus hissed, sounding almost like Voldemort's snake himself. The extra light wasn't enough to convince him that the Dark Lord wasn't waiting just around the corner to make his move. As soon as the younger Black let his guard down, he would strike and Regulus would be helpless. "He's just waiting!"
His breath was coming out in sharp gasps and his face was that of a man being dragged to the guillotine. "How did you know?" he demanded of Castiel. "How did you find out what I did? Why did you tell Him?!"