Saul was the first king of the Israelites (1 Sam 9:16) and was the son of a Benjamite called Kish, who was a man of substance and had a reputation for being a mighty man of valor.
Saul was taller than most, was handsome and before he became king he was shy, modest, humble, and was unable to hold a grudge against them that opposed of him being king. Yet, soon thereafter he became brave and decisive leader.
Yet in his later years he became subjects to fits of depression, paranoia, and obsession. His mental illness changed his character and it drove him to violent acts, bordering on madness, such as hurling his spear at his son Jonathan, killing the priests of Nob, believing they had conspired with David, who was the next king, and he also threw his spear at David more than once.
King Saul disobeys God’s demands and is removed from his kingship.
And Samuel said, What hast thou done? And Saul said, Because I saw that the people were scattered from me, and that thou camest not within the days appointed and that the Philistines gathered themselves together at Michmash;
Therefore said I, The Philistines will come down not upon me to Gilgal and I have not made supplication unto the LORD: I forced myself therefore and offered a burn-offering.
And Samuel said to Saul, Thou hast done foolishly: thou hast not kept the commandment of the LORD thy God, which he commanded thee: for now would the LORD have established thy kingdom upon Israel forever.
But now thy kingdom shall not continue: the LORD hath sought him a man after his own heart [David], and the LORD hath commanded him to be captain over his people, because thou hast not kept that which the LORD commanded thee (1 Sam 13:11-14).
And Samuel said, Then thou wast little in thine own sight, wast thou not made the head of the tribes of Israel, and the LORD anointed thee king over Israel?
And the LORD sent thee on a journey and said, Go and utterly destroy the sinners the Amalekites, and fight against them until they be consumed.
Wherefore then didst thou not obey the voice of the LORD, but didst fly upon the spoil and didst evil in the sight of the LORD?
And Saul said unto Samuel, Yeah, I have obeyed the voice of the LORD and have gone the way which the LORD sent me and have brought Agag the king of Amalek and have utterly destroyed the Amalekites.
But the people took of the spoil, sheep and oxen, the chief of the things which should have been utterly destroyed, to sacrifice unto the LORD thy God in Gilgal.
And Samuel said, Hath the LORD as great delight in burnt-offerings and sacrifices, as in obeying voice of the LORD? Behold, to obey is better than sacrifice, and to hearken than the fat of rams.
For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the LORD, he hath also rejected thee from being king (1 Sam 15:17-23).
Saul even went farther away from God, he sought advice from a witch (1 Sam 28:7-25).
God didn’t kill Saul, but from then on he lost battle after battle until finally, instead of being captured and tortured by the Philistines he fell upon his own sword (1 Sam 31:4).
The story of King Saul is in 1 Samuel, chapters 9-31.