For he did put his life in his hand, and slew the Philistine, and the Lord wrought a great salvation for all Israel: thou sawest it, and didst rejoice: wherefore then wilt thou sin against innocent blood, to slay David without a cause? (1 Samuel 19:5)
Jonathan tactfully proved to Saul that he had no reason for slaying David, by reminding him that he had every reason to appreciate David’s loyal service.