THE PLT RCVD A WX BRIEFING BFR STARTING A NGT CARGO OPN IN KNOWN ICING CONDS. BFR THE FLT, A LINE SERVICEMAN NOTICED ICE ON THE ACFT'S WINGS & SUGGESTED ITS REMOVAL, BUT THE PLT DECLINED. APRX 45 MIN AFTER TKOF, THE PLT RPRTD TO ARTCC THAT A DE-ICE BOOT HAD 'BLOWN' & HE WAS HAVING DIFFICULTY CTLG THE ACFT. HE SAID THE BOOT WAS 'GOING BACK AND FORTH' & THAT FULL RGT AILERON WAS REQUIRED TO KEEP THE WINGS LEVEL. ALSO, HE RPRTD A CONTINUOUS LOSS OF ALT & SAID THAT FULL RGTAILERON WOULD NOT KEEP THE ACFT FM TURNING TO THE LEFT. SUBSEQUENTLY, THE ACFT CRASHED ON DESCENDING TERRAIN WHILE IN A SLIGHTLY NOSE DOWN, LEFT WING LOW ATTITUDE. MUCH OF THE ACFT (INCLUDING MUCH OF THE LEFT WING) WAS DEMOLISHED BY IMPACT & FIRE. PROP SLASH MARKS INDCD THE ENGS WERE PROVIDING PWR. MAINT RECORDS HAD NO INFO OF REPLACEMENT, REPAIR OR MODIFICATION OF THE DE-ICE BOOTS IN THE PREV 10 YRS. THE PLT'S WIFE INDCD HE HAD PREVIOUSLY BEEN PRESSURED TO FLY ACFT WITH DISCREPANCIES. SIGMETS GOLF 3 & 4 WARNED OF OCNL SEVERE MIXED ICING IN CLOUDS & PRECIP.