The pilot-in-command's allowing the aircraft's airspeed to decrease below the single-engine minimum control speed (Vmc) resulting in a stall/spin condition. Factors contributing to the accident were the pilot-in-command's allowing the left engine to be shut down as well as his allowing the aircraft's airspeed to decelerate below the manufacturer's recommended intentional one-engine inoperative airspeed. A third factor was the aircraft's low altitude at the stall/spin entry which precluded a successful recovery.