A state ordered execution, in accordance with God's law is not "murder".
The law was not intended to punish a first, trivial offense. It was intended to punish repeat, hardened offenders (which you would have known, if you had actually read it). By the time the exasperated parents decided to take the offender to court, he would more than likely be at least in his teens, and well on his way to a life of crime. Add to that the fact that the parents had to run it past the village elders, so the "child" wouldn't be punished for some minor misbehavior.
You trivialize (if not deliberately misrepresent) the law when you imply that it was given to punish an 8 year-old who lipped off to his parents for the first time.