What does 1 Samuel 19:5 mean?

'He risked his life to fight the Philistine, and the Lord brought a huge victory for all Israel. You saw it and were glad. So why would you sin by killing an innocent man like David for no reason?' 1 Samuel 19:5 - Modern Text Bible

(Jonathan appeals to Saul's sense of justice, pointing out that killing David would be both wrong and senseless.)