David spared Saul's life at Ziph because Saul was the Lord's anointed. According to Jewish belief, Saul was appointed by God as the first king of Israel, and anointed with oil by the prophet Samuel. David, who was anointed by Samuel as Saul's successor, respected the sacredness of the anointing and did not want to harm Saul, despite being pursued by him. David believed that it was not his place to harm the one whom God had anointed.