We do not have moral systems because they are imposed on us. We have them because we are social beings. We evolved as small-group primates through millions of years of evolution. In order for such social groups to function, each member had to have evolved in him or her a set of innate social/moral feeling, conscience, instinct or "hard wiring" in order for the group to survive. This is true with most mammals and all primates.
If you do a favor for someone but that person does not bother to return the favor when you need it, it is normal to feel angry. It has been observed that a male chimp can get angry enough to attack the one who failed to return the favor. That is our sense of both fairness and justice. The Alpha male's dedication to protecting the group is another. His controlling the male juveniles is still another. Mothers caring for their young also.
Our moral systems are only a specific and agreed upon manner of expressing our social nature so that our enlarged groups can operate more efficiently. They are not made and imposed by "God's" but evolve through a process of natural selection termed "social evolution" in book "The Last Civilization."
Name any moral law and let's see if I can't come up with innate social source of it . . .