Calvin on why Samuel was angry

“Many grave thoughts seem to have presented themselves at once to Samuel and disturbed his mind, when he reflected upon the dishonour which might be heaped upon the name of God, and the occasion which the rejection and deposition of Saul would furnish to wicked men for blaspheming God. For Saul had been anointed by the ministry of Samuel, and he had been chosen by God himself from all the people, and called by Him to the throne. If, therefore, he was nevertheless deposed, it seemed likely that so much would be detracted from the authority of Samuel and the confidence of the people in his teaching, and, moreover, that the worship of God would be overturned, and the greatest disturbance ensue; in fact, that universal confusion would burst upon the nation. These were probably the grounds upon which Samuel’s great indignation rested.”—Calvin.

Keil, C. F., & Delitzsch, F. (1996). Commentary on the Old Testament. Peabody, MA: Hendrickson.