Another major story arc of 1 Samuel is the rise and fall of Israel's first king. Amongst the many stories in the Bible, Saul's kingship, for me, ranks among the saddest. Saul's kingship was almost doomed from the start. He was opposed by certain factions in Israel before he even began his reign, and let's not forget that strange incident where Saul is hiding on the day of his coronation and God has to tell Samuel where to find him! There are massive failures in Saul's leadership and even his personal character and when he tries to make things right, he finds it is too late. The latter portion of his kingship is spent in fearful paranoia over David, who he knows will be the next king.
I think the key to understanding the tragedy of Saul's kingship is the book of Judges. By the time Samuel appears on the scene, Israel has become a wayward nation. There is no faithfulness, there is no love of God in the nation. God wanted to rule over his people as their loving king, but the people wanted to be like the other nations, following in their ways of power, riches, and war. We could say that in Saul, God gave the Israelites the king that they deserve.
Perhaps this is at the heart of why God regretted making Saul king?