1 Samuel 22:19
And Nob, the city of the priests, smote he with the edge of the sword, both men and women, children and sucklings, and oxen, and asses, and sheep, with the edge of the sword. And Nob, the city of the priests, smote he with the edge of the sword, both men and women, children and sucklings, and oxen, and asses, and sheep, with the edge of the sword.
This belongs to the history after David killed Goliath, when Saul kept David with him and would not let him return to his father's house. Then Jonathan's soul was knit to David's and Jonathan loved him as his own soul, and Jonathan and David made a covenant as brethren. Jonathan gave David his robe and garments even to his sword and bow and girdle, and David was entrusted with important responsibilities which he carried out with modesty and won the affection of the people and the royal household.
As king Saul had Nob, the city where the priests lived, destroyed by Doeg at Saul's order, with men and women, children and infants, and oxen and asses and sheep killed with the sword. Those put to death were the priests of the Lord and a multitude of righteous persons, including children and infants. The slaughter happened after Doeg informed on Ahimelech for helping David, because the priests had let David escape, and Saul acted in maddened, envious rage to satisfy that rage. There was no command from God for it, for Saul was under Satan's control and guidance, governed by his own passions in a cruel, presumptuous and daring act.
The act stands opposite God's command to destroy the Amalekites utterly because their iniquity was full, which Saul refused out of false compassion and spared what was devoted to destruction. The contrast exposes the perversity of a human heart that has refused God's guidance, sparing where God commands judgment and destroying without mercy where God commands nothing. It was an outrage by the king Israel had chosen that filled all Israel with horror, done only after the manner of the kings of other nations who did not fear God. It left Israel with the ark in their midst but with the priests of whom they had inquired slain with the sword, bringing dread over what would come next, and standing as a living warning to Israel of the crimes a ruler may commit in rebellion against God.
The massacre marked extreme guilt and left a load of guilt that scourged Saul's conscience continually without repentance. Because of it God left him and did not answer him when he later inquired about war with the Philistines.