Deuteronomy 17:20
That his heart be not lifted up above his brethren, and that he turn not aside from the commandment, to the right hand, or to the left: to the end that he may prolong his days in his kingdom, he, and his children, in the midst of Israel.
The king was to keep God's law and word continually before him and beside him on his throne, and to obey it fully without turning aside to the right hand or to the left. He was not to let his heart be lifted up above his fellow Israelites, but to remain humble toward them. Humble, faithful obedience was the condition for lengthening his reign and securing its continuance to himself and his children, bringing prosperity and preservation of life. A king whose heart is lifted up and is not right with God trusts his own strength and judgment instead of continually trusting God and seeking His wisdom, and can lead the people away from God into rebellion. David stood firm while he held fast to the word, but sullied his reputation when he forsook it; Solomon at first sought to govern by these statutes but began his apostasy by violating God's plain commands and trusting himself; Josiah kept this rule of obedience though born to a wicked king.
The Law for the King
The verse lays down God's law for the king, requiring him to keep God's law and Word before him and beside him on his throne and to obey it fully without turning aside to the right hand or to the left. It makes keeping all the words of the law in full obedience the king's duty, holding fast its doctrines rather than departing from the commandment in either direction. Along with obedience it requires humility of heart, that his heart be not lifted up above his brethren but that he remain humble toward his fellow Israelites and not regard himself as above them. Earthly royal honor tempts a ruler to exalt himself, yet the verse forbids pride above fellow Israelites and binds humility to obedience without turning aside.
The king under this law was to be God's vicegerent, with God Himself still recognized as head of the nation and His law enforced as the supreme law of the land. Like Moses, the seventy elders, and the rulers and judges, the king had no authority to make laws for the nation but only to enforce the laws God had given. Keeping that order was the condition of Israel's continued existence as a nation, with the principles of the state unchanged though God foresaw Israel would want a king. The passage foretells a monarchical form of government for Israel and reserves to God the right to choose the king, with Israel respecting God's authority by leaving the selection entirely to Him. The choice under that divine right fell upon Saul, a son of Kish of the tribe of Benjamin.
God's foretelling that Israel would have a king did not mean that choosing a king was according to His will. When the people refused His wise counsel He permitted them to follow their choice. Samuel was instructed to grant their request while warning them of God's disapproval and the consequences of having a king.
Faithfulness and Unfaithfulness
Humble, faithful obedience to the commandment without turning to the right hand or to the left is the condition on which the king may prolong his days in his kingdom, he and his children in the midst of Israel. Keeping all the words of the law in humility and full obedience, with the heart not lifted up above his brethren, is the ground of assured prosperity and of preservation of life and reign for himself and continuance for his children. This is the same condition set in David's charge to Solomon to keep the charge of the Lord, to walk in His ways and to keep His statutes, commandments, judgments, and testimonies as written in the law of Moses.
While the king keeps the Word beside him on his throne and holds fast its doctrines he is immovable. David did this and stood firm, but when he forsook its doctrines he sullied one of the fairest reputations. Wherever the will of God is violated by nations or by individuals a day of retribution comes. Setting aside the wisdom of God for the wisdom of man and adopting human invention or device violates this requirement and leads to ruin, leaving lies as a refuge to those who turn from inspired men and from those who spread the Word before them praying God to shed light upon it.
A king whose heart is lifted up and is not right with God trusts his own strength and judgment instead of continually trusting God and seeking His wisdom and guidance. Such a ruler, exalted by earthly royal honor, can lead the people away from God and cause them to rebel. Solomon knew these instructions and for a time desired and sought to govern by them according to God's statutes given at Sinai. But his apostasy began when he violated the Lord's plain commands, increasingly trusting his own strength instead of God's guidance and blessing. Josiah followed this rule of obedience despite being born to a wicked king and facing pressure to follow his father's example, and by choosing what was right God could use him as a vessel of honor.