THE PLT WAS CLEARED FOR AND EXECUTED A RWY 17 ILS APCH AT NIGHT. PRIOR TO BEGINNING THE APCH HE WAS INFORMED THAT VIS WAS 1/8 MILE. MINIMUM FOR THE APCH WAS 1/2 MILE. AT THE MIDDLE MARKER THE PLT RATED PAX IN THE RIGHT SEAT TOLD THE PIC THAT THE RWY WAS NOT IN SIGHT. THE PAX SAID HE HAD ACTIVATED THE RWY LIGHTS WHEN THE APCH WAS STARTED. TWO PAX, BOTH PLTS, REPORTED THAT THE MISSED APCH WAS INITIATED. ONE RECALLED NOTHING MORE. THE OTHER RECALLED A CRUNCH. TERRAIN AROUND THE ARPT WAS FLAT. THE ACFT STRUCK A TREE APRX 12 FT ABOVE THE GROUND AT A DISTANCE OF 2100 FT FROM THE RWY APCH END THRESHOLD ON A RELATIVE BEARING OF 230 DEGS. AT THE TIME OF IMPACT THE ACFT WAS IN A RIGHT TURN ON A HEADING OF 345 DEGS. THE MISSED APCH PROCEDURE CALLS FOR A CLIMB TO 1700 FT THEN A CLIMBING LEFT TURN TO 3000 FT. FLAPS WERE FOUND IN THE RETRACTED POSITION.