During the feudal period of Briton, an issue rose up among the people that threatened to tear the island nation apart. The king would set laws, but then would ignore them himself. The king placed himself above the law, which the latin term Rex Lex came to signify (lit. King is Law). Open rebellion ensued and a revolution began. In the end, King John was forced to sign the Magna Carta, thus placing himself under the law: Lex Rex (meaning Law is King).
Deuteronomy 17 is clear: when God appoints a king over His people, that individual must be in submission to God’s laws. Matter of fact, the king is ordered to have his own copy of the law and to read them every day (vss18-20). This doesn’t sound very special in today’s day and age, only because we usually have several copies ourselves, but keep in mind: printing was not invented yet, and to own your own book was a big matter.
The king is not above the law, but instead the Law (God’s Word) is above the king!