Turn it around the other way, Theland. If a supernatural authority is required for people to behave decently - which is the essential point of your question - how come all non-religious people are not violent, lawless people? This is unquestionably the case.
Moreover, what percentage of murderers claim a religious faith? Is it really reasonable to say that a religious faith stops murderers?
As it is, it seems to me that your trotting out an oft-used argument against so-called ASHs without thinking through whether your contention can possibly be true.
All authority is ultimately underpinned with force, something no Christian could possibly disagree with given their god, and if self interest, ethics and inherant disposition are insufficient to stop you and you flout the laws that society has deemed necessary and are caught, you will be punished in the way the law decrees.