Ron drew his wand. Just because he couldn't think of a name - although he was running through the list of known fugitives from the war and trying to match his features - he was growing increasingly certain that the reason he recognized this man was because he had served Voldemort.
He probably should have simply tried to trace him, to find a way to track him and leave him be. Facing down an unexpected adversary in an unfriendly alley wasn't the most reasonable thing in the world. But the thought of just letting the man walk off made him angry. "Demanding, yeah," he scoffed, his wand ready but still aimed more or less at the ground. "That's because it's my job to drag scumbags like you off to Azkaban where you belong. Most people appreciate it if we ask questions first." But even if Ron didn't know who he was, he knew what he was. And if he was wrong ... well. A false arrest did less harm in his mind than leaving someone like this on the street. "What's your name?"