THE PLT INITIATED A DOWNWIND LANDING ON A GRASS RWY THAT SLOPED MOSTLY UPHILL, BUT BEGAN SLOPING DOWNHILL AT ITS DEPARTURE END. A WITNESS NEAR THE APCH END OF THE 2150 FT RWY WATCHED THE ACFT TOUCH DOWN BEYOND ITS MIDPOINT, THEN DISAPPEAR OVER ITS CRESTED END. A WITNESS NEAR THE DEPARTURE END HEARD THE ACFT & 'TURNED TO WATCH THE PLANE TAKEOFF.' HE STATED THAT THE ACFT WAS HEADED TOWARD TREES & THAT IT LOOKED LIKE THE PLT WAS TRYING TO PULL UP. SUBSEQUENTLY, THE ACFT COLLIDED WITH THE TREES ON DOWNSLOPING TERRAIN, CRASHED & BURNED. ONE PASSENGER, THE PLT'S SON, SURVIVED THE CRASH,BUT DIED LATER DUE TO THE INJURIES & BURNS. AN EXAM OF THE CRASH AREA REVEALED THAT THE ACFT HAD HIT A BARB WIRE FENCE. ONE OF THE PROP BLADES HAD A BARB WIRE RADIUS NICK MARK ON ITS LEADING EDGE. PREVIOUSLY, THE PLT HAD STATED THAT AT THISARPT, HE PREFERRED TO ALWAYS LAND UPHILL, REGARDLESS OF THE WIND. THE WIND AT THE TIME OF THE ACCIDENT WAS REPORTED TO BE FROM 215 DEG AT 10 GUSTING 15 KTS.