DURING ARRIVAL FOR AN ILS RWY 21R APCH, THE ACFT ENCTRD ICING CONDS FOR ABOUT 9-1/2 MIN. AS THE ACFT WAS VECTORED FOR THE APCH, THE SEATTLE ARTCC CTLR USED AN EXPANDED RADAR RANGE & DID NOT PROVIDE PRECISE PSNG OF THE ACFT TO THE FINAL APCH COURSE. THE FLT CREW ATMTD TO CONT ON A STEEP, UNSTABILIZED APCH FOR A LNDG. RECORDED RADAR DATA SHOWED THAT THE ACFT WAS WELL TO THE RGT OF THE ILS COURSE LINE & WELL ABOVE THE GLIDE SLOPE AS IT PASSED THE OUTER MARKER/FINAL APCH FIX (FAF). IT DID NOT INTERCEPT THE LOCALIZER COURSE UNTIL IT WAS ABT 1.5 MI INSIDE THE FAF. ALSO, IT WAS STILL WELL ABOVE THE ILS GLIDE SLOPE WERE RECORDED ALT DATA WAS LOST WHEN THE ACFT WAS ABT 2.5 MI FM THE ARPT. THE TWR HAD CLOSED, BUT THE CTLR SAW THE ACFT IN A HIGHER THAN NORMAL RATE OF DSCNT IN A WINGS LEVEL ATTITUDE. BFR RCHG THE RWY, THE ACFT NOSED OVER & CRASHED IN A STEEP DSCNT. THERE WAS EVIDENCE THAT ICE HAD ACCUMULATED ON THE AIRFRAME, INCLUDING THE HORIZONTAL STABILIZERS, WHICH MAY HAVE RESULTED IN A TAIL PLANE STALL. (SEE: NTSB/AAR-91/06)