No, a lawyer does not need to report a crime that has already occured--even one as serious as murder; not only would doing so, if the lawyer found it out  in the course of represenation, as your question suggests, be a violation of attorney client privilege, but as a general matter, while someone may not actively obstruct justice, there is no requirement for people to come forward, whether lawyer or not, with knowledge of crimes.However, a lawyer is under an affirmative obligation to come forward if he believes his client is planning or about to commit a new serious crime. So if the lawyer knows or believes there is a new murder in the offering, the lawyer has to disclose that to the authorities, even if the knowledge came in the course of representation.