Where do legal systems come from?

In the years before post-structuralism made it to the provinces, my red-cheeked eagerness to be “pre-law” led me into an undergraduate philosophy of law course.  I had no idea what law had to do with philosophy, and also I had done very badly in the freshman philosophy survey course; but I convinced myself this was something I should do.  In the event, the course would alter my trajectory in life.  Not in the obvious way, as in, learning something that I believed in.  Far from it: I rejected then and reject now much that I read that semester.
The teacher was adjunct at that time.  He ultimately didn't win tenure in the philosophy department after years of slogging in the teaching trenches.  He took his graduate philosophy degree with him to law school, went into law practice, had a family, and continued to feed his pedagogical hunger by teaching college kids at night (and still does, I believe). 
He began the first 3-hour night class by posing a simple hypothetical.  Assume, he posed, the most heinous acts imaginable against an infant child – abuse, mutilation, the works.  Now, he asked, can you give me any scenario under which such acts are justifiable?  Further, is there any possible way that such acts are not always and everywhere wrong?  Such questions invoke the doctrine of natural law.  As it developed, the entire semester’s readings reflected, in some way, his interest in natural law.  I didn’t accept it then and don’t now, but I was angry enough to work to refute it. 
The idea, not very politic in the academy of the early-1980’s, was that immutable values and truth existed and that it was possible, with philosophical inquiry, to get at those values and truths.  The answer to the infant child hypothetical is that the acts are always and everywhere wrong, independent of anyone’s cognizance or appreciation of those acts.  That is, there is Wrong and Right.  It takes little intellectual bootstrapping to set these up as reflections of a deity who is the benchmark for the standards, and I suspected that the teacher was a religious person.  At all events, he was something of a throwback as of 1985, though similarly conservative minds would soon come out swinging with fear-mongering attacks on fashionable philosophical ideas.
About a year later, post-structuralism established beachheads in the second and third tier universities like mine, and those of us invited to the party were excited.  This was the whole relativism debate, with critics on the right attacking the universities for promoting godlessness and for abdicating any adherence to “values.”  These attacks were reminiscent, to me, of the philosophy of law instructor’s tenets, though they were stated less forthrightly, less baldly religious in tone. The arguments were not pointedly about absolute right and wrong – natural law – so much as the tenets of “western” culture: truth, beauty, love, those kinds of things, and the great books from classical times to the 19th century that were argued to traffic in these things.
Post-structuralism, variously called post-modernism, deconstructionism, critical theory, or none of these, depending on whom you consult, had to do with validating different perspectives in the evaluation of cultural artifacts (art, books, music, movies, etc.).   That is, people bring different perspectives to the table when interacting with cultural artifacts, and their reactions and interpretations are all of equal worth.  In practice, the critic refers to cultural artifacts other than the one under scrutiny to develop the critical perspective on the work.  Thus, the critic might refer to the works of Marx in explicating a novel by Dickens.  In that instance, we would say that the “perspective” of the critic is leftist.  It often happened, in post-structuralist criticism, that leftist theorists, Freud, and feminists were the chief references -- the critical touchstones -- and this tended to enrage (and engage) the political right and practically demand that post-structuralism be damned as abhorrent. 
Post-structuralism’s seeming dominance in the academy of the mid and late 1980’s galvanized conservatives both inside and outside of the academy.  In the end, Post-structuralism won the cultural wars, but only by becoming defanged and irrelevant.  The critical move of validating differing interpretations of cultural artifacts is now de rigueur and meaningless, a sort of bad warm-up joke before a keynote speech.  Writers clear their throats by acknowledging multiple perspectives on a subject then go on to discredit everyone’s views but their own, in the time-honored tradition of criticism. 
I have long agreed with Stanley Fish, a founder and still the most clear-eyed post-Structuralist, that “post-Structuralist philosophy” is a contradiction in terms.  Post-structuralism doesn’t tell you anything that really matters, for all its deconstructionist glee in knocking down the monuments of Western culture with expositions of their (take your pick) phallocentrism, dialectical materialism, anal retentiveness, and all the rest.  All it really tells you is that people argue by their lights and really do believe in things very deeply.  It should come as no surprise that the critic who uses Marx to explicate Dickens reliably votes Labour; it would be quite surprising to learn otherwise or to hear that same critic praise Margaret Thatcher’s policies.  I doubt that such a critic teaches you anything truly new about Dickens’ work, though you probably do learn a lot about how people on the left wish you to view Dickens; perhaps that is something worth knowing.  Philosophy, however, it is not. 
I long cherished the hope that post-structuralism would provide the answer to my philosophy of law teacher's hypothetical – can heinous acts against a young child ever be justified or ever be right – and only reluctantly did I put post-structuralism aside as inadequate to the task.  I agree with the prof. that it’s impossible for a society to tolerate such harms, so deeply ingrained in us is the sense of their outrageousness and so grievous is their real damage.  The problem for me has always been the project that inevitably follows this conclusion: the construction of a system of rights and wrongs with perfect predictive power – that is to say, a rigid moral system and, finally, a rigid legal system based on that moral system: the doctrine of natural law.  Since I believe, deeply, that any such system is both impossible to construct in a truly pluralistic society and, were it somehow accomplished, profoundly oppressive, I have looked elsewhere for answers.