THE PLT HAD FLOWN TO THE ARPT TO PERFORM AEROBATIC MANEUVERS AT A FLY-IN. HE HAD A LOW ALTITUDE WAIVER TO PERFORM DOWN TO 500 FT AGL. BEFORE THE FLIGHT, HE TOLD SEVERAL SPECTATORS HE WAS GOING TO DO A MANEUVER RIGHT AFTER TAKEOFF. WITNESSES REPORTED THAT THE ENG SOUNDED NORMAL DURING THE TAKEOFF. SHORTLY AFTER LIFT-OFF, THE PLT TURNED ON HIS AIRSHOW SMOKE. AT AN ESTIMATED ALTITUDE OF 200 TO 300 FT, HE EXECUTED A RAPID RIGHT ROLL THAT WITNESSES DESCRIBED AS A SNAP ROLL, REPORTEDLY, THE ACFT ROLLED APRX 90 DEG PAST 1 COMPLETE ROLL, THEN THE NOSE PITCHED & THE PLANE CRASHED IN A NEAR VERTICAL ATTITUDE. AN AIRLINE PLT (OWNER OF A BELLANCA DECATHALON) WAS ONE OF THE WITNESSES THAT DESCRIBED THE MANEUVER AS A SNAP ROLL. ACCORDING TO HIS DESCRIPTION, THE PLT USED INSUFFICIENT RUDDER TO STOP THE ROLL AFTER COMPLETING 1 TURN.