I have no way of knowing whether a supernatural higher law exists or not. If there is a supernatural higher law then it certainly will be imposed on me. But so far as I can ascertain the laws that I have encountered were strictly human in origin. Of course there are the laws of physics, etc but that is another story.
So when I say “moral people” I mean people whose actions I approve. I am using my own standard of things that I like and things that I don’t like based on my own human desires and cultural experiences.
When we feel guilt, remorse or shame, I believe it is probably due to our desire to fit in and get along with others. This desire to get along and fit in helps us survive. We need the help of other people to thrive in this world. Greed, lust, hunger, fear and other desires sometimes overpower our desire to get along. We all use stereotypes to make quick judgments about others and we identify with people who are like us and fear those who are different from us. The Nazi’s were not unique except in their particular genocidal methods. I don’t think the Nazi’s were wrong because they failed to meet an absolute moral standard. I think they were wrong because I would not have wanted to suffer at their hands.
If I can find someone that agrees with me on how I feel about certain behavior then the two of us can try to influence others to behave the same way. After a while we may have a large group of people that follow our “Moral Code.” We can set up a system of reward and punishment to enforce these “morals.” Our Code may be quite useful and sensible and in total agreement with Biblical teaching, but there would be nothing supernatural about it.