שמואל א, פרק י״ח, פסוק ט׳

A sharp turning point occurs in the relationship between the King of Israel and his young servant. Where there was once fondness, suspicion and hatred now take root. The primary approach among commentators is that Saul began to look at David with an evil eye, viewing him through a lens of deep hatred [רש״י, רד״ק, מצודת ציון]. However, this hostility went beyond a mere hateful glance. It involved a continuous, obsessive thought process. Saul did not just watch David; he focused all his mental energy on finding a way to destroy him [מצודת ציון, רלב״ג].

The root cause of this sudden shift was a deep fear. Saul suspected that David was the very man the prophet had spoken of when warning that the kingdom would be torn away and given to a neighbor who was better than him [מלבי״ם]. Driven by this paranoia, Saul began actively plotting David's death. He sought various methods to eliminate him, ranging from indirect schemes to violence by his own hands, always waiting for a moment when David would let his guard down. A clear example of this occurred when a distressing spirit from God came over Saul, causing him to speak in confusion. As David played music to soothe him, Saul planned to use the distraction as an opportunity to strike him dead with his spear [רלב״ג, מלבי״ם].