Albert Barnes Commentary Ezekiel 9:6

"slay utterly the old man, the young man and the virgin, and little children and women; but come not near any man upon whom is the mark: and begin at my sanctuary. Then they began at the old men that were before the house." — Ezekiel 9:6 (ASV)

Begin at my sanctuary - The first to be punished were those who had brought idolatry nearest to the holy place. The “ancient men,” that is, the 25 men who had stood with their backs to the altar (Ezekiel 8:16) were the first to be slain.