What did Samuel do to Agag king of the Amalekites, whom Saul had spared?

Answer: Hacked him to pieces

Samuel said, "As your sword has made women childless, so your mother will be childless," and hacked Agag to pieces at Gilgal.

“Then Samuel said, “Bring me Agag king of the Amalekites.” Agag came to him cheerfully, for he thought, “Surely the bitterness of death is past.” But Samuel declared: “As your sword has made women childless, so your mother will be childless among women.” And Samuel hacked Agag to pieces before the LORD at Gilgal.”— 1 Samuel 15:32-33 (BSB)