A Dinwiddie County woman fatally stabbed her 6-year-old son after learning that her husband had left her and wouldn’t agree to sign away his rights to their child, a prosecutor charged yesterday. Although no clear motive was established yesterday in the Nov. 24, 2006, slaying of Shawn A. Futrell, prosecution witnesses testified that Julie E. Futrell was deeply distraught and scared after learning her husband had essentially moved out of their Dinwiddie home two days before Thanksgiving.