We report reversible switching between Ohmic and negative differential resistance states at a threshold voltage in sub-100-nm diameter sulfurized antimony selenide nanowires. We show that threshold switching in our nanowires arises due to high non-equilibrium free carrier concentrations resulting from impact ionization of carriers from defect states traceable to sulfurization and surfacedangling bonds. Threshold switching is suppressed because of inhibited carrier generation at air-passivated defect states or at high temperatures due to thermally induced carrier depletion from deep states which preempts impact ionization. Such non-linear phenomena would be important for designing phase-change memories, thermoelectric devices, and sensors using pnictogen chalcogenidenanowires.