David has the opportunity.

Saul, the king who has hunted him for years, who threw spears at him, who massacred the priests who helped him, is asleep in a cave, completely vulnerable. Abishai sees it immediately: God has delivered your enemy into your hand. One stroke of the spear. It will be over.

David refuses. He takes Saul’s spear and water jug as proof he was there, and leaves Saul alive. The opportunity that would end his suffering, David lets walk away.