Regulus paused, and something dark crossed his expression as well. "You know, the Dark Lord said something very similar to me, once. He underestimated me as well. If you're going to do the same, then I have no trouble slipping back out of the country and leave you and your friends to beat yourselves to death over and over trying to take him down," Regulus said stiffly. It was one thing to have Sirius assume he was untrustworthy. It was entirely another to have Sirius deem him as not worth the effort.
He didn't answer the rest, still bristling over the insult to what little pride he had left. It hadn't been easy to do this, and Sirius seemed to think it had been a walk in the park for him.