THE FLT WAS AN AIR TAXI CARGO FLT CARRYING TRUCK PARTS. AFTER TAKEOFF, THE TWR ASKED THE PLT IF HE WAS IN DIFFICULTY AND THE PLT REPLIED 'ROGER' AND REQUESTED A CHANGE OF FLT PLAN. THE ACFT WAS SEEN FLYING ERRATICALLY JUST ABOVE THE TREE TOPS. THE ACFT DESCENDED ALMOST VERTICALLY INTO A RESIDENTIAL AREA. THE REAR OF ONE HOME WAS IGNITED BY THE IMPACT AND 2 OCCUPANTS OF THE HOUSE WERE FATALLY BURNED. POST ACC INVESTIGATION REVEALED NO ENG MALFUNCTION BUT THERE WERE DISCREPANCIES IN THE LOAD MANIFEST AND THE ACFT WT & BALANCE. BEFORE TAKEOFF A CO EMPLOYEE WATCHED THE PLT SECURE THE CARGO WITH ROPES NETS & CLIPS BUT DID NOT HELP BECAUSE HE 'DIDN'T KNOW WHAT THE PLT WAS DOING.' IMPACT 'AMAGE PRECLUDED DETERMINATION OF THE ORIGINAL PLACEMENT AND SECURITY OF THE CARGO AND ANY POSSIBLE SHIFTING PRIOR TO IMPACT. THE NET WEIGHT ON THE CARGO MANIFEST DID NOT INCLUDE THE WT OF THE TRAYS TO HOLD THE PARTS. THIS WAS AN OMISSION OF 150 LBS. CO PERSONNEL SAID NO WT & BALANCE INFO WAS LEFT WITH THEM WHEN THE ACFT DEPARTED.