In this past week's torah portion (Genesis) Cain decides to kill his brother Abel and is duly punished by God. There have been some bloggers that have suggested that without the torah we would turn to a life of murder, robbery, debauchery etc. These people claim that we need the torah to help us act as a just society.
So if that is the case, why was Cain punished by God? The torah was yet to be given, so he didn't break any rules. According to this line of thought, he was just acting on a natural uinstinct that God just imbued in him.
I think it is sad that the obvious answer, that common sense is what tells you killing is wrong, is unfortunately not so obvious to some. I am not saying that the torah has not helped improve morals, but to say that we would have none without it just seems ludicrous.