The problem is that we must judge ourselves. People have made as many claims about what the authority above the law (if such exists) says as about what the law says.
I prefer to look at the law as part of a system which controls the power of the state. Our way of doing things has been judged by history to be extremely good compared to other ways. Whether it will continue to be so, whether we can adapt to whatever new circumstances we must face, is always unknown.
The problem is that we must judge ourselves. People have made as many claims about what the authority above the law (if such exists) says as about what the law says.
You are correct in that humans are stuck on earth with nothing but other humans to justify and provide earthly judgement. But to rephrase Blackstone's presumption - there is an authority that most of the people SUBSRCIBE TO, outside of the law. In that context, making law is relatively easy.
My belief is that civilizations that believe and act as though they are the final authority in defining right from wrong are not durable.
Societies are complex indeed. I believe there is a spiritual battle going on as well, a literal battle over our souls.
I think all human endeavors are flawed. After all, we are human ;-)