This question comes from several verses found in the book of 1 Samuel. Saul was the first king of Israel. The nation of Israel was coming out of a time when God had used various Judges to deliver them and rule over them. That system however proved to be unsuccessful because the Israelites continued to sin. In 1 Samuel 8:4-7 the Israelites demanded a king so that they could be "as all the other nations." The problem was that all the others nations were not God's people, they were not set apart as the Israelites were. The people of Israel were set apart to be different from the other nations and called to steer clear of the same sin and idolatry the other nations participated in. Despite God's continual struggle with and deliverance of Israel during their past times of disobedience, they still had not learned their lesson. Perhaps you and I can learn from their mistakes to avoid repeating them?
King Saul was the first king of Israel and was appointed by God (1 Samuel 9:14-16) and we see from the start that Saul may had some flaws. In 1 Samuel 10 Saul is received as king, but when they went to appoint him they couldn't find him because he was hiding in some supplies stashed nearby (1 Samuel 10:22). We see Saul with a similar act of cowardice in the story of David and Goliath. In that story king Saul should have been leading the Israelite army against the Philistines, instead he was just as afraid as the rest of them. Flaws and all though Saul was the one appointed by God. This wouldn't be the first or the last time God chose someone with less than desirable qualities and used them for his purpose. Thankfully for all of us the Bible is full of people with character flaws and checkered past that God used in mighty ways. Saul's story however is a good example of what happens when we dismiss God's commands and don't turn back from our sin.
By the time we get to 1 Samuel 13 we see a major failure on Saul's part. As Saul and his troops were preparing to fight the Philistines they were waiting on Samuel to come and offer a fellowship and a burnt offering. Being impatient at Samuel's delay Saul offered the offerings himself (1 Samuel 13:9-12). This was as foolish decision as we see in verse 13. It was the job a the priest to offer such offerings, not the king. Saul had just violated the law of God and Samuel told him it was going to cost him his kingship and informs him that God has already appointed a new king that will take his place.
From this point we begin to see a downward trajectory in Saul's life. The next act of disobedience comes a few chapters later. Saul and the Israelites were to go and attack the Amalekites and completely destroy them and all they had (1 Samuel 15:3). Instead the Israelites chose to keep out some of the more valuable and useful items of the Amalekites (1 Samuel 15:9). Saul also allowed the king Amalek to live when he was to be destroyed. Saul makes his best attempt to explain away his disobedience, but Samuel stops him and tells him God has rejected him as king (1 Samuel 15:12-23). It is in the next chapter that we see the evil spirit come upon Saul. The Spirit of the Lord had previously been on Saul, but his disobedience to the Lord has now resulted in the Lord's Spirit leaving him (1 Samuel 16:14). The rest of the verse may be troubling for some because it says "an evil spirit sent form he LORD began to torment him," We have to remember here though that Saul brought this on himself. These actions were the result of Saul's sin and disobedience. It was the absence of the Lord that left Saul open to this attack. The Lord is the only thing that gives anyone the power to overcome evil. Without the Lord we are all subject to attack. God is a very loving and patient God (2 Peter 3:9) but he will not strive with man forever (Genesis 6:3). We see evidence of this in the New Testament as well. In Romans 1:28 Paul speaks of those who would rather live in sin than to listen to God. Paul says that "God delivered them over to a worthless mind to do what is morally wrong." He delivers them over to their evil desires because they have rejected Him, and if someone rejects God, only evil remains. This is what I believe the Lord did to Saul. He removed His protection from Him and turned him over to his evil desires. As Saul's story continues in 1 Samuel 18-19 we again see a mention of this evil spirit on Saul and we see his behavior and life continue to spiral downward.
Saul's life isn't the only mention of this type of thing occurring. A similar thing happens in the story of Samson. The Spirit of the Lord came upon him early in his story, but at the end we see that "...the Lord has left him." (Judges 16:20) This too was because Samson was disobedient to the Lord. Sometimes however, God may turn us over to evil for a while to help us to realize how much we need Him so that we would repent and come back to Him. Paul speaks of such a case in 1 Corinthians where there is a man who is caught in sin. What advice does Paul give the church of Corinth, "turn that one over to Satan for the destruction of the flesh, so that his spirit may be saved in the Day of the Lord." (1 Corinthians 5:5) It seems as though Paul thought some ultimate good would come from that situation in the end even though the man was "turned over to satan".
The story of Saul is a serious reminder of the importance of remaining faithful to the Lord and keeping ourselves free from sin. You may be in a time in your life that God is trying to get your attention and that's a good thing because "...the Lord disciplines the one He loves and punishes every son He receives." (Hebrews 12:6) If God is trying to get your attention, listen to Him! If there is sin in your life then repent. Don't fall into the same trap as Saul. Thankfully for us, in every way Saul failed as king, Jesus succeeded. While the people of Israel demanded a king so they could "be like all the other nations" God knew that they needed a king unlike any earthly king so that they would be just the opposite, children of His kingdom, not children of an earthly kingdom. Jesus is the true king and through Him we can gain forgiveness of our sin and escape the path of Saul if we choose to follow Him and put our faith in Him. "But God, who is rich in mercy, because of His great love that He had for us, made us alive with the Messiah even though we were dead in trespasses. You are saved by grace! Together with Christ Jesus He also raised us up and seated us in the heavens, so that in the coming ages He might display the immeasurable riches of His grace through His kindness to us in Christ Jesus. For you are saved by grace through faith, and this is not from yourselves; it is God’s gift — not from works, so that no one can boast. For we are His creation, created in Christ Jesus for good works, which God prepared ahead of time so that we should walk in them." (Ephesians 2:4-10) Jesus has offered you the invitation to His kingdom, don't reject it! "The time is fulfilled, and the kingdom of God has come near. Repent and believe in the good news!" (Mark 1:15)