Saul said to David, Behold, my elder daughter Merab, her will I give you as wife: only be valiant for me, and fight Yahweh's battles. For Saul said, Don't let my hand be on him, but let the hand of the Philistines be on him.
This chapter illustrates the fact that the godly often suffer through no fault of their own. It shows too that God causes even the worst intentions of the ungodly to strengthen the godly (cf. Psalms 7:12-16; Romans 8:28). We see here that the selfishness of the ungodly can produce irrational behavior (e.g., paranoia, 1 Samuel 18:12, and schizophrenia, 1 Samuel 18:11; 1 Samuel 18:17), and it leads to their ruin. I am not implying that this is the only cause of these mental problems. If we allow jealousy to take root in our hearts, it will devour us like a cancer. We should desire God’s glory, as Jonathan did, rather than our own glory, as Saul did.