God created Laws. That's where you get murder to be wrong, not from man but from God.
Laws and regulations have been in place since one person was more powerful than another. The more powerful could always say "I eat first." or similar.
This practice grew into setting up sets of rules with punishments for non-compliance
Mankind has been around a lot longer than Christianity in fact many thousand of years.
When people first came down from the trees life was a lot harder and the only way to survive was by cooperating and working together, even with out any thought of laws there would have been ways of acting that would go against the greater good of the group so it would only have been natural to have a kind of code of conduct that helped the greater good.
Slowly these would have become the norm.