I think it's Regulus Black. We all know that Sirius' brother got in "so far" with the Death Eaters, panicked at what he was being asked to do, and tried to get out. Of course since we're not meant to like anyone in the Black family we're lead to believe that he was a coward who ran for it. But what if he didn't? What if he tried to take a piece of Voldemort down with him?