Why did king Solomon sin?

Answer 1

God was lenient to Solomon for the sake of his father David, who had been a man after God's own heart. David had sinned too, but was entirely broken and repentant when he realised his sin.

Also: When God was describing Himself to Moses, He said that He, "[...visits] the lawlessness of fathers on the sons, and on the sons of sons, to the third and fourth generation." Exodus 34:7

This isn't about God punishing innocent children for the sins of the parents - it's about children inheriting the evil ways of their parents. God has given parents a tremendous amount of power to shape the character, beliefs, and habits of their children.

If you live a godless, selfish life, it's very likely that your children will follow your example. By the same token, if you "worship in spirit and in truth" John 4:24 and diligently teach your family the Word of God (Deuteronomy 6:4-9), your children are much more likely to decide to live for God, also.

This is exactly what happened with David, Solomon, and Solomon's successor Rehoboam: David had many virtues, but unfortunately seems to have neglected his children in some ways. I doubt Solomon spent much time being a father, either; for most of his reign he was much too busy collecting women like Baseball cards.

Solomon repented near the end of his life and wrote Ecclesiastes (and likely Proverbs) at that time in an attempt to prevent the next generation from following his foolish example. (Though Solomon was wise in understanding, he was very foolish in his actions for much of his life.) He worried about his successor, in particular (Ecclesiastes 2:18-19):

"Yes, I hated all my labor which I had done under the sun; that I must leave it to the man who shall be after me. And who knows whether he shall be wise or a fool? Yet he shall have rule over all my labor in which I have labored, and acted wisely under the sun. This is also vanity."

When Solomon died and his son Rehoboam took the throne after him, all the people of his kingdom gathered together and basically said (1st Kings 12:1-5), "Don't treat us like slaves the way your father did, and we'll be loyal to you."

Common sense suggests that a man cannot rule over a people without at least some cooperation from them. Also, Rehoboam didn't need all the forced labour that his father did, because Israel was at the height of its power; David had already given Israel military security, and Solomon already finished the temple, the royal palace, and great fortifications, and the economy was strong.

But Rehoboam wanted to live a life of self-glorification and -gratification, just like his father had, so he gave an incredibly foolish and proud answer to the people (1st Kings 12:10-11):

"...My little finger shall be thicker than my father's loins. And now my father loaded you with a heavy yoke, and I will add to your yoke. My father has whipped you with whips, but I will chastise you with scorpions."

After that, most of the tribes of Israel rebelled against Israel, just like God had foretold in His judgement against Solomon.

So, God was merciful to Solomon, but no one can say that He was unjust to his son - Rehoboam clearly deserved what he got.

"What shall we say then? Is there not unrighteousness with God? Let it not be!

For He said to Moses, 'I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.'"

Romans 9:14-15

Answer 2

He told Solomon that part of the kingdom would break away (as later happened); and Hadad the Edomite was sent to harass him (1 Kings 11:11-14).