A newly-developed fast neutron detector revealed a close relationship between the loss of pre-disruptive runaway electrons and the plasma disruption in KSTAR ohmic plasmas. It is observed that a burst of photoneutrons is generated exactly before the start of thermal quenches, indicating a bunch of runaway electrons which had existed before the disruption impacts the first wall at the time. The loss of runaway electrons could be identified also as a decrease in the measured electron temperature, forming a typical two-stage thermal quench trace. From the MHD pattern in the neutron signal during a low-q disruption, it could be identified that pre-disruptive runaway electrons are localized in the plasma, especially on the q = 2 drift surface. These new findings suggest the pre-disruptive runaway electrons might play an important role in the plasma disruption mechanism.