THE PLT INITIATED AN ILS APCH IN BELOW MINIMUM WEATHER CONDITIONS SHORTLY AFTER NIGHT FALL. HE WAS GIVEN A LOW ALT ALERT, BUT CONTINUED THE APCH. HE ANNOUNCED A MISSED APCH, AND INDICATED PROBLEMS CLIMBING TO ALT. HE THEN DECLARED AN UNSPECIFIED EMERGENCY, AND REQUESTED VECTORS TO A LARGER ARPT, IN SPITE OF WORSENING WEATHER CONDITIONS. HE REACHED AN ALT OF 2100 FT, BUT COMMENCED ANOTHER ILS APCH DESPITE PREVIOUS DIFFICULTIES. RADAR DATA INDICATES EXCESSIVE MANEUVERING TO INTERCEPT THE INBOUND COURSE, WITH WIDE VARIATIONS IN GROUNDSPEED AND ALT. FOLLOWING A STEEP CLIMB AT A LOW GROUNDSPEED, THE ACFT DISAPPEARED BELOW RADAR COVERAGE. WITNESSES INDICATED IRREGULAR ENGINE SOUNDS AS THE ACFT MADE A TIGHT LEVEL TURN AT LOW ALT PRIOR TO IMPACTING TREES IN A NEAR LEVEL ATTITUDE. ON-SCENE EVIDENCE SHOWS POWER WAS BEING PRODUCED FROM BOTH ENGINES UPON IMPACT. POST-IMPACT FIRE CONSUMED THE WRECKAGE.