The present article, which is the result of several years of focus and research on the permissibility of capital punishment as a form of retribution, employs a descriptive-analytical approach to address a fundamental question: What are the serious objections, based on numerous Quranic verses, against the most severe form of punishment—i.e., the deprivation of life—not in the context of Gisas (retributive justice) in private cases but rather in the context of Tazir (discretionary punishment) and Hadd (fixed punishment) in the public domain? Does the principle of proportionality between crime and punishment, as a core doctrine of criminal law, hold in such cases? We argue that, based on the unequivocal declaration of the Quran, the unjust or inappropriate killing of a single soul—even if the individual is a criminal—is akin to the killing of all humanity. Therefore, the legitimacy of capital punishment must be explicitly stated in the Quran, which, in the author's view, it indeed is. Our hypothesis in this study is that, given the extraordinary sanctity that God has assigned to human life as His most honored creation, it is only natural that whenever capital punishment is deemed necessary, the Quran must explicitly authorize it. Our findings confirm that the Quran does permit capital punishment, but exclusively within two exceptional cases: (1) Gisas—retribution for murder, and (2) Ifsad fi al-ard (corruption on earth) when it leads to homicide as a crime against the public order. Additionally, the legitimacy of killing an assailant who intends to take the life of an innocent person falls under the category of legitimate defense, which is a natural right rather than a legislated one. Even in the realm of natural sciences, this principle is upheld by Newton’s Third Law of Motion, which states that every action has an equal and opposite reaction. This fundamental principle, widely known as the law of action and reaction, is universally accepted, leaving no room for dispute regarding the legitimacy of self-defense.