Deuteronomy 17:20

20 by not being overbearing toward his fellow Israelites, and by not deviating even a bit from the commandment. If the king does all that, he will ensure lasting rule in Israel for himself and for his successors.

Deuteronomy 17:20 Meaning and Commentary

Deuteronomy 17:20

That his heart be not lifted up above his brethren
On account of his office, the dignity of it, considering that he was subject to the law of God, and accountable to the Lord for all his actions:

and that he turn not aside from the commandment, to the right hand or
to the left;
not in the least deviate from the law of God in the whole of his conduct, and particularly in the exercise of his kingly office:

to the end that he may prolong his days in his kingdom;
ruling well according to the laws of God being the way to rule long:

he and his children in the midst of Israel;
this shows, as Jarchi observes, that if his son was fit for the kingdom, he was to be preferred to any other man; for though it was elective, yet to be continued in the same family, provided they walked in the ways of the Lord, and observed his laws.

Deuteronomy 17:20 In-Context

18 Instead, when he sits on his royal throne, he himself must write a copy of this Instruction on a scroll in the presence of the levitical priests.

19 That Instruction must remain with him, and he must read in it every day of his life so that he learns to revere the LORD his God by keeping all the words of this Instruction and these regulations, by doing them,

20 by not being overbearing toward his fellow Israelites, and by not deviating even a bit from the commandment. If the king does all that, he will ensure lasting rule in Israel for himself and for his successors.