Albert Barnes Commentary 1 Samuel 15:11

"It repenteth me that I have set up Saul to be king; for he is turned back from following me, and hath not performed my commandments. And Samuel was wroth; and he cried unto Jehovah all night." — 1 Samuel 15:11 (ASV)

It grieved Samuel - "Samuel was angry, or displeased," as Jonah was (Jonah 4:1), and for a similar reason. It seemed a slur on his prophetic office.

He cried unto the Lord - With the wild scream or shriek of supplication (see 1 Samuel 7:8–9; 1 Samuel 12:18). The phrase and the action mark Samuel’s fervent, earnest character.