PLT WAS AUTHORIZED BY ACFT OWNER TO TEST FLY THE ACFT AROUND THE TRAFFIC PATTERN WITH A LOCAL FBO OWNER ON BOARD. PLT MADE 3 FLTS, LAST OF WHICH CULMINATED IN THE ACCIDENT. ON THE FIRST AND THIRD FLTS, UNAUTHORIZED PASSENGERS WERE ON BOARD. FBO OWNER STATED PLT PERFORMED A 'COUPLE ROLLS' DURING THE SECOND FLT WHEN HE WAS ON BOARD. ON THE THIRD FLT, SEVERAL WITNESSES STATED, PLT 'BUZZED' THE RWY AND MADE A WIDE CLIMBING TURN TO A WIDE DOWNWIND NORTH OF THE RWY. ONCE LEVEL, BTW 700 AND 1000 FT, PLT BEGAN A SERIES OF LEFT AILERON ROLLS. NUMBER OF ROLLS VARIED AS REPORTED BY WITNESSES, HOWEVER, ALL STATED THAT DURING THE LAST ROLL THE ACFT HESITATED NEAR THE INVERTED POSITION, THE NOSE DROPPED AND ACFT CONTINUED TO ROLL INTO THE GROUND. THREE FLT INSTRUCTORS STATED THEY ADMONISHED THE PLT ON SEVERAL OCCASIONS IN THE PAST 'NOT TO PERFORM AEROBATICS IN THE AIRCRAFT' ESPECIALLY SINCE WING PODS WERE INSTALLED.