Sherlock kept the decision of meeting the murderer in private from John.When Sherlock showed up at the set place,to his surprise,he saw John standing infront of him who was supposed to go to Sarah's.It turned out that John was held hostage and turned into a human bomb,and the real killer is Jim Moriarty,whom they met during the first case as Molly's boyfriend.How will Sherlock and John get out of the dangerous situation with bombs and snipers threatening them,we will have to wait for BBC to tell us the story in season two.