A.T. Robertson Commentary Mark 6:27

"And straightway the king sent forth a soldier of his guard, and commanded to bring his head: and he went and beheaded him in the prison," — Mark 6:27 (ASV)

A soldier of his guard (σπεκουλατορα). Latin word speculator. A spy, scout, lookout, and often executioner. It was used of the bodyguard of the Roman emperor and so for one of Herod's spies. He was used to do errands of this sort and it was soon done. It was a gruesome job, but he soon brought John's head to the damsel, apparently in the presence of all, and she took it to her mother. This miserable Tetrarch, the slave of Herodias, was now the slave of his fears. He is haunted by the ghost of John and shudders at the reports of the work of Jesus.