1 Samuel 20

"Saul boiled with rage at Jonathan. 'You stupid son of a whore!' he swore at him. 'Do you think I don’t know that you want him to be king in your place, shaming yourself and your mother? As long as that son of Jesse is alive, you’ll never be king. Now go and get him so I can kill him!'
'But why should he be put to death?' Jonathan asked his father. 'What has he done?' Then Saul hurled his spear at Jonathan, intending to kill him. So at last Jonathan realized that his father was really determined to kill David." v.30-33
Saul was still trying to secure his throne for future generations even though he had already been told his dynasty would end with him. Even worse, he was trying to do this by sinful human means because he knew he would get no help from God. Jonathan could have made a move to become the next king by killing his rival, his best friend, but he bypassed this opportunity because of his love for both God and David.

Taken from the NLT Life Application Study Bible