At about 1020 Mountain time, the Part 135 commercial cargo flight collided with flat open terrain.Visual conditions prevailed; the aircraft was operating on an IFR flight plan. The aircraft was destroyed by impact damage and a post-crash fire. The Airline Transport pilot-in-command and another deadheading pilot were fatally injured. The flight originated from Salt Lake City, Utah, about one hour prior to the accident and was destined for Friedman Memorial (SUN), Hailey, Idaho. After executing the RNAV (GPS) Runway 31 approach to SUN, the flight was cleared to land while still in instrument conditions. There were no further communications. The pilot of a Cessna Citation 525, who flew the same approach to SUN about 20 minutes prior to the accident aircraft, reported his aircraft was picking up moderate mixed ice on the wings and windshield.