The S&P 500 and Nasdaq joined the Dow in negative territory in midday trading as banks raised some red flags. The Dow fell 393 points, or 1%, while the S&P 500 and the Nasdaq Composite were down 0.4% and 0.3%, respectively. The KBW Bank index, designed to track the performance of the large U.S. national money centers, regional banks and other institutions, was down 3.3%, as Goldman’s CEO flagged trading revenue issues and Ally Financial noted consumer stress.