Oh... bloody jealousy!

God decided to grant David the powers that Saul had becuase he was divine, and a born king.
"23: And it came to pass, when the evil spirit from God was upon Saul, that David took an harp, and played with his hand: so Saul was refreshed, and was well, and the evil spirit departed from him." David was better off than Saul, and was then prefered by God. This obviously provoked jealousy on Saul. Which made him act wrongly. He envied David deeply because he was a great friend of his son's and he was granted power by God, because he was victorious and was the hero and was his daughter Michal was taken as his wife. And suddenly all that Saul wants is to kill David and overtake him. His own daughter will not let him do that and either will all the people who are with David. At the end he is the author of his own death.
Jealousy is the worst enemy you could ever have. The only one who suffers is yourself because the rest of the people are not affected. It leads you to do things that have no right argument and probably would not make things better. Jealousy is a complex feeling, it is a mixture of anger, unsatisfaction, sadness, impotence, rancor, self-insecurity and other... You feel desperate and there is absolutly no right reason for it. The only cure for that jealousy plague is to let it be and just accept it.