The defect chemistry of various lithium and sulfur point defects in the solid electrolyte Li_4P_2S_6 in its planar arrangement in the P  3¯1m space group is studied by means of total energy calculations within density functional theory. We show that the formation of Li Frenkel-pairs is the dominant internal defect reaction which provides the charge carriers for ionic conductivity. External sulfur related defect equilibria can be excluded as compensation mechanisms for lithium defects. Moreover, we find that charged lithium interstitials exhibit negative formation energies, pointing to the instability of the electrolyte against metallic lithium. This tendency is supported by total energy calculations that predict a highly exothermic reaction between Li_4P_2S_6 and metallic lithium to give Li_2S and Li_3P. The extent of this interfacial instability is such that we observed substantial reactivity even within static calculations on representative interface models. Depending on the surface orientation, a more or less ordered interphase is formed. The formed interphase shows similarities to Li_2S, which has been suggested to act as a passivating layer that inhibits further reaction between the thiophosphate and metallic lithium.