Charles Ellicott Commentary Judges 20:39

"And the men of Israel turned in the battle, and Benjamin began to smite and kill of the men of Israel about thirty persons; for they said, Surely they are smitten down before us, as in the first battle." — Judges 20:39 (ASV)

And when the men of Israel retired. —This merely repeats with more graphic details the fact already mentioned in Judges 20:31. The “when” should be omitted, and from “Benjamin began” to the end of the next verse is parenthetic.