On the implication of afterlife punishment

"Morality comes from god" or "Without god you can't be moral" - common claims from people who believe in god but doesn't want to understand how morality in a society develops overtime. Simple questions like, "What would you make of a situation where god believers commit crimes?" or "What about the pedophilia, slavery and genocide sanctioned by abrahamic god?" or "What prevents someone to start a mass killing spree in the name of his god?" are enough to repudiate such flimsy assertions. Not to mention a number of problems you face pondering on 2nd horn of Euthyphro Dilemma when you discuss Divine Command Theory.

But yet this is a point I wish to discuss in this post, to give an answer to a question repeatedly asked by theists in order to garner support for the necessary existence of god. A typical version of this question would be,

"If there was no god to punish the criminal in afterlife then people like Hitler would emerge and the victims wouldn't get justice. To ensure justice and balance in crime and punishment, even if earthly life appears insufficient to punish criminals, they will surely get punished in afterlife."

Notice that in this single argument, a theist is justifying the existence of god, afterlife, punishment of hell & reward in heaven. Also notice that this is a statement to argue both - people like Hitler would emerge in human society frequently if there was no god (because it implies that people would be immoral without god hence commit crimes), and it is a statement to establish the concept of divine justice - ergo God necessarily exist.

First & foremost, the concept of "justice in afterlife" generates enough mental comfort (wishful thinking) for the believer to believe it. Thus this concept is a belief and the believer believes it is necessary to believe in this belief, for this has an emotional factor, a strong motivation derived from experiencing apparent lack of injustice in human life. A belief in belief (necessary beliefs) of this kind among the members is roughly inversely proportional to the overall law and order situation of a society.

The equivalency between crime and punishment is arbitrary. Which means we tend to constantly reevaluate our judgement on which can be considered as crime and how adequate the punishment is for a crime. It seems that the justice system in human society has been changing its form from retaliation to rehabilitation which denotes the evolutionary progression of morality and human rights in our society. But that's beside the point. Society developes as more and more of its members become law abiding citizen where neither the laws nor the out-laws violate human rights. This is fundamental factor for a society which struggles to progress.

A society would plunge into medieval darkness if it allows for some of its members to establish totalitarian authority. Throughout history we could see clashings of oppressed with the oppressor and emergence of societies with greater freedom and rights. A society upgrades when its members make sure there remain less criminals and abusers which ultimately ensures a collective boost in overall happiness. Thieves, mass murderers, rapists, robbers - all these anti-social elements are a risk to the overall happiness of the citizenry. We see a continuous improvement of laws, police force, justice and punishment system in response. We also see dictators fall, authoritarian regimes collapse, people revolt. Thus the members of a society make sure to get rid of factors that harm their progress. And if they fail, we see degradation of society and standards of life.

Fear of god didn't prevent any dictator from mass murdering people. On the contrary, most of them justified their crimes as necessary evil and frequently declared that god is on their side who approved of their actions. See Gott Mit Uns of Hitler, Bush and Muhammad.

Thus the guaranty of punishment of a criminal is a necessary factor which members must ensure for the sake of a peaceful society. I'd argue that wishful thinking of an afterlife punishment not only generates a false satisfaction but also it effectively demotivates the believers to think of a better way in order to improve the "earthly" justice system which eventually harms the society. The concept fails in its totality. It depends on the subjective type and level of faith and degree of supernatural fear one has in his mind.