I Samuel 22:18-19
vs. 19 Also Nob, the city or the priests, he struck with the edge of the sword, both men and women, children and nursing infants, oxen and donkeys and sheep with the edge of the sword.
 
Saul's refusal to obey God and wipe out the Amalekites in chapter 15 of I Samuel sets the stage for this heinous crime against the Lord. Turns out that Saul is capable of utterly destroying a community, sparing no living thing. But in his mad rebellion, instead of wiping out God's enemies, he becomes one and begins to exterminate God's people. Like so many before and after him, Saul has gone from being God's anointed to being an enemy of righteousness. Rather than walking humbly with God, honoring Him in everything, Saul becomes great in his own sight. He was very impressed by what he saw when he looked in the mirror. He surely concluded that his anointing was his own invention. A "self-made man" as it were.