The failure of the pilot to execute the published emergency procedures pertaining to configuring the airplane for single engine flight, which would have allowed him to maintain minimum controllable airspeed (Vmc) and level flight. The pilot's failure to maintain minimum controllable airspeed (Vmc) led to a stall and subsequent Vmc roll at a low altitude. Contributing to the accident was the operator's inadequate pilot training in the single engine flight regime, and the loss of power from the left engine for undetermined reasons.