The theory of karma should not be confused with so-called "moral justice" or "reward and punishment." The idea of moral justice, or reward and punishment, arises out of the conception of a supreme being, a God, who sits in judgment, who is a law-giver and who decides what is right and wrong. The term "justice" is ambiguous and dangerous, and in its name more harm than good is done to humanity. The theory of karma is the theory of cause and effect, of action and reaction; it is a natural law, which has nothing to do with the idea of justice or reward and punishment. Every volitional action produces its effects or results. If a good action produces good effects and a bad action bad effects, it is not justice, or reward, or punishment meted out by anybody or any power sitting in judgment on your action, but this is in virtue of its own nature, its own law. This is not difficult to understand. But what is difficult is that, according to karma theory, the effects of a volitional action may continue to manifest themselves even in a life after death.
~ Walpola Rahula