Concise Bible Dictionary:

This word does not occur in the Old Testament except in the margin. In three places persons are pointed out as “captain of the guard,” who in the margin are called “chief of the executioners or slaughter men” (Gen. 37:3636And the Midianites sold him into Egypt unto Potiphar, an officer of Pharaoh's, and captain of the guard. (Genesis 37:36); Jer. 39:99Then Nebuzar-adan the captain of the guard carried away captive into Babylon the remnant of the people that remained in the city, and those that fell away, that fell to him, with the rest of the people that remained. (Jeremiah 39:9); Dan. 2:1414Then Daniel answered with counsel and wisdom to Arioch the captain of the king's guard, which was gone forth to slay the wise men of Babylon: (Daniel 2:14)). In Solomon’s day Benaiah the chief of the army was called to fulfill this office (1 Kings 2:25,34,4625And king Solomon sent by the hand of Benaiah the son of Jehoiada; and he fell upon him that he died. (1 Kings 2:25)
34So Benaiah the son of Jehoiada went up, and fell upon him, and slew him: and he was buried in his own house in the wilderness. (1 Kings 2:34)
46So the king commanded Benaiah the son of Jehoiada; which went out, and fell upon him, that he died. And the kingdom was established in the hand of Solomon. (1 Kings 2:46)
), though doubtless the “chief” had others under him that actually carried the king’s word into execution, unless the persons were of high rank. In Mark 6:2727And immediately the king sent an executioner, and commanded his head to be brought: and he went and beheaded him in the prison, (Mark 6:27) Herod Antipas called to an executioner, or one of his guard, to behead John the Baptist.

Strong’s Dictionary of Greek Words:

of Latin origin; a speculator, i.e. military scout (spy or (by extension) life-guardsman)
KJV Usage: