How did Cain know it was wrong to kill? The commandments were not written yet...how could he be punished if he had no idea he had done anything wrong...it must not have been "wrong" to him...or he would not have done it.
How about why you feel something is wrong and an convincing explanation instead of "cause I said so or else"?
If people agree with the explanation...it would stand to reason..they would not have to be forced to comply.
Why was the commandments not explained fully as to why they are wrong..and what the long term consequences are from breaking them?