THE ACFT IMPACTED A HILL IMMEDIATELY AFTER TAKING OFF. THE WRECKAGE WAS LOCATED 8,000 FT BEYOND THE DEPARTURE END OF RWY 28 & 1,800 FT LEFT OF THE RWY CENTERLINE EXTENDED. THE ACCIDENT SITE WAS APPROXIMATELY 55 FT ABOVE THE AIRPORT ELEVATION. WITNESSES OBSERVED THE ACFT MANEUVERING AT LOW ALTITUDE DURING A SNOW STORM. THE FLT HAD ORIGINALLY DEPARTED JACKSON,WY EARLIER IN THE DAY, BUT DIVERTED TO RIVERTON DUE TO AN OIL LEAK CAUSED BY THE OIL CAP NOT BEING IN PLACE. THE FLT REMAINED IN RIVERTON APPROXIMATELY 2 HOURS WHILE THE ACFT & ENG WAS BEING CLEANED. PRIOR TO DEPARTING JACKSON WITNESSES STATED THE PLT WAS "SOMEWHAT RUSHED & ANXIOUS TO DEPART JACKSON" & "THE MAN HAD A BAD CASE OF WHAT YOU MIGHT CALL GET-HOME-ITIS, & HE WAS GOING WHETHER IT WAS VFR OR IFR." THE ACFT WAS APPROXIMATELY 253 LBS OVER MAX GROSS WEIGHT.BOTH INBOARD CLOTH SEAT BELTS SEPARATED AT THE JUNCTURE WHERE THEY PASS THROUGH THE METAL GUIDES LOCATED ON THE SIDES OF EACH SEAT.