one rejects the possibility of divine and/or afterlife retribution, why should people recieve leniency in this life? It seems to me that people who do not believe in a final reckoning would desire a system in which the punishment is on par with the crime, yet I find that to not be the case. Why?

Aetheists (or anyone who does not believe that wrongdoers will be punished by a higher power) who believe in a less harsh system of punishment seem to me to want to deny justice in favor of mercy. Justice would require that person A pays the price for all of the damage done to person B. Mercy understands that in many situations there cannot be a sufficient price paid by the wrongdoer to satisfy the demands of justice and so mercy grants a smaller price. This seems counterintuitive to me because to me, mercy only works without excluding justice if there is a divine reckoning. Since the assumption of this long question is no divine reckoning, it would seem to me that a lenient system of punishment for wrongdoers is merciful and unjust to the wrongdoer and merciless and unjust to the victim. Since on this earth far more of us will be victims than will be wrongdoers wouldnt it make sense that on this earth the wrongdoers recieve a just but unmerciful punishment so that the limited time each of us has on this earth is more fully enjoyed by more people?