Roy Lindley DeVecchio, the retired Federal Bureau of Investigation supervisor charged with murder, walked out of court a free man this morning after prosecutors dropped all charges against him in the wake of new evidence that the government’s main witness changed her account and may face perjury charges. The lead prosecutor, Michael F. Vecchione, announced the decision to dismiss all charges this morning in State Supreme Court in Brooklyn, where Mr. DeVecchio, 67, was charged with helping a Mafia informer commit four murders in the 1980s and early 1990s. The trial, which began last month, was upended this week after...