It is true, of course, that law is a tool for shaping behavior. So it is instrumental. Yet it is not primarily or essentially a tool to be used on people, behaviors, or institutions. Even though law can be a tool and we often treat it as tool, its essence, its nature, its first order purpose is not as a tool to be wielded. (Note here the passive voice, which points out a secondary problem: who is the righful wielder of the tool and who decides what end is to be sought?)
The late Harold Berman put it this way:
It is widely accepted in our law schools that law is essentially something that is made by political authorities, including legislators, judges, and administrators, to effectuate their policies; that law is essentially a means of social engineering; that law is essentially a pragmatic device, an instrument, used by those in power to accomplish their will. Of course, law is all that. But it is not solely that-- it is not essentially that. What is omitted from the prevailing view is a belief that law is rooted in something bigger than the people who hand it down -- that law is rooted in history and in the moral order of the universe.
HAROLD J. BERMAN, The Crisis of Legal Education in America, in FAITH AND ORDER: THE RECONCILIATION OF LAW AND RELIGION 333-34 (Scholars Press 1993) (cited in REDEEMING LAW, p. 24).
In short, the question is whether the essence, the nature, the primary purpose of law is instrumental: that is, something created by those in power to accomplish particular ends. The answer is no: it is not primarily, essentially, naturally a mere tool. When law is reduced to a pragmatic human artifact, it loses its very nature as something rooted, as Berman says, in the moral order of the universe.