Perovskite solar cells, mainly based on the prototypical CH3NH3PbI3, have recently emerged as a disruptive technology in the solar cell industry. Despite the great successes in the exceptional power conversion efficiency, many underlying mechanisms involving the structures and performance of the perovskite solar cells remain elusive, such as the role of the methylammonium cation and the optimization of the passivation technique. These hinder the formulation of a universal design principle that can guide us to rationally improve the perovskite solar cell performance. In addition, the notorious stability issue in perovskite solar cells prevents the solar cell from wide industrial application. In this manuscript, we closely examine the cation rotation and the passivating layer at the halide perovskite surface in the context of the stability issue. An experimentally-proved passivating agent iodopentafluorobenzene (IPFB) is used as an example. The ab initio investigation is performed via constraining/relaxing the cation rotation at the perovskite surface and manipulating the coverage of IPFB. Many existing reports tend to agree that the methylammonium cation rotation could be beneficial to the outstanding performance of the perovskite solar cell due to the ferroelectric effect. On the other hand, the passivating agents such as molecular additives tend to universally improve the device stability. In this study, our calculation suggests that: 1) the methylammonium cation rotation could undesirably accelerate the water infiltration process at the perovskite surface; 2) the passivating layer can counter-intuitively facilitate the water interaction with the perovskite surface; both of these could severely degrade the solar cell stability. Therefore, we emphasize on the double-edged sword effects of cation rotation and additive passivation on the stability of the perovskite solar cells. This study facilitates the convergence to the fundamental understanding of the organic-inorganic perovskite solar cells and is beneficial for the rational design of the halide perovskite-based optoelectronic devices.