What if he meant that human nature is human nature ~ the good stuff, and the bad stuff, too. It could be that he is saying precepts developed over time have been developed to make communal living possible. That we each experience anger or rage or grief or greed or shame and etc, and that our tendency will be to justify whatever our responses were. Which people do when they say things like, "Well, you know what? It was a crime of passion. I couldn't help myself and the other guy deserved it."
And then, whatever system of law we have devised assigns degrees of guilt or innocence and assigns punishment.
We say that to justify war and reparation, right? Someone did thus and so and tried to take my stuff so we killed them. All of them, except for some that we missed. And we think whoever is left should pay for the cost of destroying them because we won, and we all agree they never should have done that.
That we each experience, in varying degrees, all the rawness of every emotion. And that, if we hope or intend to create societies of more than one person, we need to understand that social mores set up and working over time make that possible.
So, however foolish we think the rules are in times of peace and plenty, we only have those good times of stability because of the rules, so don't be messing around trying to think that we know better than the tried and true things that got us all to this point.
It could be that he is saying a person who has thought this through understands this and so, acts independently to obey the laws established by his communal group.
Not do not judge, but don't be so arrogant as to believe you know best when you, individually, have only been here one lifetime that you know of.
So in a way, it is do not be arrogant. Do not assume you know a new and better way because we no longer understand the reasons the old ways were adopted and found worthy. We are all the same in the core challenges we confront, and that is multiplied when we are trying to live together.
So the truth of human nature would be self interest, right? And that would be a myriad of things. Sifted through time, we find the rules that best work to make it possible for the greatest number to survive with honor.
Cedar