A new fluid model for runaway electron (RE) simulation based on fluid description is introduced and implemented in the magnetohydrodynamics (MHD) code M3D-C1, which includes self-consistent interactions between plasma and REs. The model utilizes the method of characteristics to solve the continuity equation for the RE density with large convection speed, and uses a modified Boris algorithm for pseudo particle pushing. The model was employed to simulate MHD instabilities happening in a RE final loss event in the DIII-D tokamak. Nonlinear simulation reveals that a large fraction of REs get lost to the wall when kink instabilities are excited and form stochastic field lines in the outer region of the plasma. Plasma current converts from RE current to Ohmic current. Given the agreements with experiment on RE loss ratio and mode growing time, the simulation model provides a reliable tool to study macroscopic plasma instabilities in existence of RE current, and can be used to support future studies of RE mitigation strategies in ITER.