THE BOOKS OF KINGS. Why they were written?

The aim of the Books of Kings is to demonstrate that  the covenant relationship with God is not just in the physical realm, it has to be in harmony with the mental, emotional, and spiritual realms.
The writer sets out to demonstrate that although Israel stood in covenant relationship with God, most of her Kings had rejected and outraged the covenant obligations.
The Kings of both Judah and Israel are reviewed while they were passing, and as far as possible, they were treated contemporaneously. The spiritual worth of each King is determined by comparison with two Kings of former years. These two Kings were: King David who held closely to the covenant, and King Jeroboam of Israel who forsook the covenant. Comparison in this way shows whether a given king "walked in the ways of Davis his father" or "in the ways of Jeroboam the son of Nebat."
The writer, evidently, found that on this basis very few of the Kings of Israel or Judah kept the covenant with God. Notable exceptions were the King Asa (1Kings 15), King Jehoshaphat (1Kings 22), Hezekiah (2Kings 18-20) and Josiah (2Kings 22-23), and even these had some defects.
David was the ideal example, he was the more closely role model than any. His parting advice to his son Solomon was that he should keep God's commandments (1Kings 2). In that loyalty to the covenant relationship lay the only hope of prosperity and peace. To depart from that way was to risk divine judgment.
Loyalty to God's covenant stemmed from Abraham, but found national expression at the time of Exodus, when Israel,  recently delivered from Egypt, meaning the forces of the flesh, stood at Mount Sinai and entered into a solemn spiritual covenant with God (Exodus 19; 24). Thereafter Israel was to be God's own people, set apart from the nations, obedient to His commandments and loyal to him.
They were forbidden to enter into covenants with other entities or other gods. Adherence to the covenant with God would result in blessings; departure from it would result in cursing and judgment.
These principles are clearly worked out in 2 Kings 17-23.
The writer traces the story of Israel's Kings from Solomon to the last king of Judah. In a sincere and honest manner he records the sad story of the rejection of the covenant by most of the rulers. The final collapse of Israel before Assyria (2Kings 17) and of Judah before Babylon (2Kings 25) was a demonstration of the Truth of the Principle underlying the Book and came as no surprise to men of spiritual discernment.
In later days the two Books of Kings remained as a warning to the remnant of God's people, and thus provided a practical lesson in the Truth that rejection of God's covenant, being a sinful and rebellious act, can only result in divine judgment.