In a small and unchanging group, in which you are forced to deal with the rest of the members for all your life, doing things to other members of the group (like murdering them) is likely to have severe repercussions, and is probably not really in your genetic advantage. Crime doesn't pay.

In larger groups, or groups with shifting populations, Crime may well pay, because if you are caught, you can just change groups - "living in the old tribe just didn't work out, and it was time to move on". Large cities provide the illusion of anonymity, so people follow their programming/instincts which tell them to cheat. Of course the anonymity is actually an illusion - we have huge computer databases to keep track of them, and a police force, a judiciary and penal system to deal with them when they commit crimes. Naturally too, they in turn feel very cheated when they get caught, and call themselves victims. After all they were only following their instincts, so why does society have the right to punish them?

Then we lock them up for 10 years or so (well, murderers anyway), and then they say "I now understand that murder is wrong". So we let them out and say "behold, they are rehabilitated, the system works". The truth is that they are getting on to 30 by this time, and in a hunter-gather society they would be getting considerably weaker than the currently maturing generation. Naturally then, they favor a society which places less emphasis on physical force. It's then obvious to them that murder is morally wrong. We like to call this wisdom.