Li-N-H compounds hold promise as novel hydrogen-storage materials with high gravimetric hydrogen densities. Because the dehydriding reaction caused by the decomposition of LiNH2 requires a higher temperature than desired, much effort has been devoted to the destabilization of LiNH2 to decrease the decomposition temperature. In particular, there has been recent experimental evidence for lowering the temperature by partial substitution of Li by Mg. However, the reason is not clear. In this study, we have employed density functional theory to investigate LiNH2 and partially Li-substituted systems aiming to understand the effects of the substitution on the destabilization of the NH2 species. K, a more electropositive element, and Mg, a more electronegative element, have been chosen as two probes to illustrate the effects. We have focused on the investigation of effects of substitutions on the N-H bond strength that is regarded as a qualitative indicator of the decomposition temperature. We have found that in both cases the N-H bonds are weakened, in particular, the Mg substitute appears to be more effective in the destabilization of the NH2. The relative strength of the metal-N ionic bonding has been found to be a key factor to explain the effects of the substitutes. These have been discussed in detail in terms of Wannier function analyses.