David has no choice but to flee from Saul, so Saul loses his best military leader. Saul’s actions damage his relationship with his son and in the chapters that follow, Saul spends so much time trying to kill David that he becomes ineffective at defending Israel against its enemies. Saul turns against those who could have been his most powerful helpers - God and David - and instead becomes his own worst enemy.
How different this story could have looked had Saul embraced God’s plan. We could avoid a lot of stress and disappointment in life if we did things God’s way instead of developing solutions based on our inadequate efforts.