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In this investigation, we have synthesized thermally stable cubic CsSnCl$_{3}$ perovskite nanocrystals (crystal size ~300 nm) with better surface morphology by the hot-injection technique. Excellent crystalline quality of these cubic nanocrystals was confirmed by high-resolution transmission electron microscopy imaging. The binding of organic ligands on the surface of the sample was characterized by nuclear magnetic resonance spectroscopy. The UV-visible spectroscopy ensured that CsSnCl$_{3}$ nanocrystals have a direct bandgap of ~2.98 eV which was confirmed by steady-state photoluminescence spectroscopy. The band edge positions calculated by the Mulliken electronegativity approach predicted the potential photocatalytic capability of the nanocrystals which was then experimentally confirmed by the photodegradation of RhB dye under visible and UV-visible irradiation. Our theoretical calculation by employing the generalized gradient approximation (GGA) and GGA + U methods demonstrated 90% accurate estimation of experimentally observed optical bandgap when $U_{eff}$ = 6 eV was considered. The ratio of the effective masses of the hole and electron expressed as D = $m_{h}^{*}/m_{e}^{*}$ was also calculated for $U_{eff}$ = 6 eV. Based on this theoretical calculation and experimental observation of the photocatalytic performance of CsSnCl$_{3}$ nanocrystals, we have proposed a new interpretation of the "D" value: a "D" value of either much smaller or much larger than 1 is the indication of low recombination rate of the photogenerated electron-hole pairs and the high photocatalytic efficiency of a photocatalyst. We believe that this comprehensive investigation may be helpful for the large-scale synthesis of thermally stable cubic CsSnCl$_{3}$ nanocrystals and also for a greater understanding of their potential in photocatalytic, photovoltaics and other prominent optoelectronic applications.