Japan’s benchmark Nikkei 225 stock index plunged 12.4% on Monday, resuming sell-offs that are shaking world markets as investors fret over the state of the U.S. economy.The Nikkei closed down 4,451.28 points at 31,458.42. The market’s broader TOPIX index fell 12.8% as selling picked up in the afternoon.Darkening the outlook for trading on Wall Street, early Monday the future for the S&P 500 was 2.4% lower and that for the Dow Jones Industrial Average was down 2.6%.A report showing hiring by U.S. employers slowed last month by much more than expected has convulsed financial markets, vanquishing the euphoria that had taken the Nikkei to all-times highs of over 42,000 in recent weeks.The Nikkei 225 dropped 5.8% on Friday, making this its worst two-day decline ever. Its worst single-day rout was a plunge of 3,836 points, or 14.9%, on Oct. 20, 1987, a day that was dubbed “Black Monday.” This Monday …