Looking at the way metaphors shape the way we think metaphorically about morality, combined with some insights from evolutionary biology helps explain why people take fairness and justice so seriously.
English has two main metaphors for morality, both of which are ultimately based on the schema of balance. In one, we more literally see acts as having weight. In the Egyptian Book of the Dead the soul of the deceased is weighed in a set of scales with a symbol of the law on the other side of the scales. In this view, justice involves either lightening the weight of evil, or adding to the weight of good.
Theravādins used this metaphor in discussing karma which can be weighty (garuka) or light (agaruka).
Early Buddhists saw karma as inescapable. This is actually what Buddhaghosa meant when he referred to the "restriction on karma" or kamma-niyāma. Mahāyānists introduced many ways to avoid the consequences of actions through religious exercises, including confession. There is a list of such practices in Śāntideva's other book, The Training Almanac (Śikṣasamucaya).
The other metaphor is more abstract and involves book-keeping. When credit and debit columns of the ledger match we say the books are "balanced". It's the same schema, but a different metaphor. In this view an evil action is a debit, or a debt. A good action is a credit. In most human societies debts have to be paid and often with interest. This why when we've done something wrong we metaphorically say that someone is owed an apology. The apology settles a debt. It balances the books.
The least sophisticated version of this is like for like (an eye for an eye). Other models allow substitutions. Which is why we think locking people up is about "paying your debt to society". Moral debts follow can be settled by various methods: confession, atonement, restitution, reparation, etc. And of course debts may be forgiven. The same Hebrew tribes that gave us "an eye for an eye" had built in mechanisms for forgiving debts as well. Later in this worldview Jesus came down to earth to settle all our debts with God and leave the books balanced. This is religious genius and has played very well with the punters. Buddhism has never been so daring in its forgiveness of moral debts.
The various Buddhist version of karma do not use this metaphor explicitly, but there is always a sense in which the rebirth one gets balances out how one has lived in this life. The metaphor is implicit - the principle at work is balancing good and evil.
Now this schema of balance and the resulting metaphors are not accidental or random. They emerge from the two fundamental features that all social mammals and birds share: empathy and reciprocity. Clearly reciprocity is more important in the idea of moral balance. At base it is simple give and take. It is why the statement "actions have consequences" seems so intuitive to us (and why it is universally recognised as a moral principle). Though reciprocity in the fullest sense requires us to recognise and respond to the needs of others, i.e. empathy.
Social animals have to practice give and take to make a group successful. Sharing of resources and making sure that even the weaker members of the group have enough is important because the evolutionary strategy of social animals is "safety in numbers". The coherence of the group is what makes it effective as an evolutionary strategy. Where those animals have a hierarchy (which is always) then being higher up the hierarchy is associated with greater privileged access to resources, but also greater obligations to the group. Groups gang up on predators, for example, and being higher up the hierarchy means being on the front line. Except in civilised humans, where are leaders are often not in the front line physically. Leaders are seen as too precious to put at risk of death in combat.
But actually "actions have consequences" is not quite specific enough for morality to work. And here we can refer to Buddhaghosa's use of the term niyāma "restriction". The consequences of actions must be appropriate to the action (bīja-niyāma) and they must be timely (utu-niyāma). By bīja-niyāma Buddhaghosa meant that a kuśala action was restricted in such a way as to have a kuśala consequence and an akuśala action had to have an akuśala consequence. Hence the image of a rice seed (bīja) giving rise to a rice plant. And by utu-niyāma he meant that consequences were restricted to arrive in the right season (utu), just as the monsoon rains come at the right time (at the end of three months of baking hot dry weather), or fruits and flowers all happen at the same time. Utu means "seasonal" and can also refer to other cyclic processes like menstruation. Buddhaghosa added another restriction which was the karma had to ripen and could not be avoided, which he called kamma-niyāma.
Where the consequence of actions are seen to be avoided we call that unfair or unjust. Where the consequences are not appropriate to the action we call that unjust. And when consequences are delayed we call that unjust. We share this basic view not just with all other humans, but with most other social animals. Buddhism does not have a unique take on morality, it just has has the same package in a different wrapper.
Now as regards kuśala/akuśala is it apparent that these do not balance out in this life. Hence an afterlife is required and a primary the function of the afterlife is exactly to provide this balance. If the world is just or the universe is moral, then an afterlife is necessary to make up for the obvious injustice that prevails in saṃsāra.
Timeliness can vary. Aṅgulimāla for example found all his karma ripening in this life (though for a mass murderer he got off very lightly). The Loṇaphala Sutta describes how someone poor in the Dharma might experience life times in hell, but someone rich in the Dharma might experience a trifling sensation in this life for the same evil action. Mostly early Buddhists saw rebirth as the fulcrum of the balance - any imbalance in how we live in this life directs our rebirth. Later views changed, especially in relation to the extent that a Buddha may intervene in this process (more and more as time goes on).
Of course Buddhists introduced the radical idea that one could escape from this cycle of actions having appropriate and timely consequences by ensuring they removed the conditions for rebirth. If one is not born, then none of these arguments apply. One is free of all these constraints and goes beyond explanation.
The acme of Buddhist debt forgiveness is the Vajrasatva mantra which is said to purify all our karma in one go. It may well give us subjective relief, but it doesn't change how society sees the balance of our actions in relation to them. the bottom line is that we are social animals and all morality has to be seen in terms of how our actions impact on others and how their actions impact on us. We all understand morality in terms of "balance" and we intuitively know when things are out of balance and we desire to see balance restored.
In other words Buddhists seem to say that we can forgive ourselves for transgressions and that will somehow magically translated into social forgiveness. It does not take too much effort to see that this is never the case in practice. Society wants to see justice done, and they don't much care if you have forgiven yourself.
This desire for moral balance can be frustrated in many ways, by the exercise of power for example, or because other demands are weightier. But the desire doesn't go away. If reciprocity breaks down, then the message we get is that our survival is threatened. The desire for justice is visceral and powerful for this reason. This is why people will kill if they perceive that it will restore the balance.
I haven't gone into how conservatives and liberals see things differently. This is another fascinating dimension of the cognitive approach to morality. But people will only read so much on the internet and this rave is already too long.
This rave is based on ideas found in John Searle's book "The Rediscovery of Mind"; George Lakoff's long essay "Metaphor, Morality, and Politics"; and Frans de Waal's book "The Atheist and the Bonobo". I highly recommend all three.