Shortly after reporting the outer marker while executing an ILS runway 18 approach in actual instrument meteorological conditions, the aircraft deviated left of the localizer centerline. At a position approximately 1/2 mile outside the middle marker and about 1/4 mile left of the localizer centerline, the aircraft descended into a wooded area on an easterly heading. The aircraft impacted the trees and then the terrain in a slightly left bank and slightly nose low attitude. Engine examination and teardown revealed no evidence of pre-impact malfunction. A flight check of the ILS runway 18 was performed the next day and indicated the system was functioning normally. Inspection of the aircraft instruments did not disclose any malfunctions. The pilot was in contact with ATC and did not report any problems before the impact occurred. All pilot and aircraft records were destroyed by post-impact fire. Pilot's total instrument flying experience was not determined; obtained instrument rating 1/3/87.