He disobeyed God's Law...
Deuteronomy 17
14When you enter the land the Lord your God is giving you and have taken possession of it and settled in it, and you say, “Let us set a king over us like all the nations around us,”
15be sure to appoint over you a king the Lord your God chooses. He must be from among your fellow Israelites. Do not place a foreigner over you, one who is not an Israelite.
16The king, moreover, must not acquire great numbers of horses for himself or make the people return to Egypt to get more of them, for the Lord has told you, “You are not to go back that way again.”
17He must not take many wives, or his heart will be led astray. He must not accumulate large amounts of silver and gold.
Read 1 Kings 9 through 1 Kings 11. (I think those are the chapters. He breaks every commandment. He buys horses and chariots from Egypt and sales them to other govenemnts (Arms dealer), he offers his own children to Molech in the fire through his many wives, he makes silver more abundant and common. He builds the temple with slave labor... It's not about assigning Solomon a bad rap, its about knowing that even somebody as wise as solomon can make huge mistakes. On that note, Ecclesiastes was written as a confession...