The theory of karma and the belief in sin and punishment are mere myths. There is really no compelling evidence in support of them. We simply believe that evil actions result in retribution and good acts result in good providence. These are just theories of convenience. There is really no valid proof of these theories.
For e.g. when we witness another person perform something bad we mentally assume that nemesis will catch up with that person. We believe that justice will eventually prevail. Similarly when we do something good, perform some altruistic act we internally assume that we gain karmic merit. Others may believe that kindness and benevolence will ultimately result in a great time in heaven or some such thing.
In reality none of this is really true. Your life is ultimately the result of choices that you make. One could be evil and still make intelligent choices and navigate through life fairly successfully. On the other hand, there are altruists who suffer because they lack basic common sense.
Simply believing that “every action results in equal and opposite reaction” is naïve.
Having said that karma & sin are mere theories and have no substance does not mean that I am advocating an immoral or evil life. Ultimately every action that we do must be based on a simple maxim “Do unto others as you would have them do unto you”.
It is likely that the theory of karma & sin/punishment play an important role in the maintenance of law in the lives of mankind.
Ultimately it is ability to maintain the highest quality of life for mankind as a whole that should be basis for any thing that we do or don’t do.