The "external policeman" is a problematic concept, because the only thing standing between a civilised society and a free-for-all, dog-eat-dog Mad Max arena is the law enforcer: he becomes the one who demands compliance to social conduct; not your conscience, not your regard for fellow human beings, not your understanding of WHY it is such a bad idea to maim and kill your neighbours.
Children go through a phase where they don't yet fully understand moral implications, but comply because authority figures threaten them with punishment. You know what happens once you remove the authority figure? Lord of the Flies.
Now, perhaps our species needed this stage in their collective cultural development at some point, but I doubt it's a permanent solution. We need something a little more sophisticated than threats of punishment to truly get along.