Matthew Poole and continuators on Judges 21:5

5 And the children of Israel said, Who is there among all the tribes of Israel, that came not up with the congregation unto the LORD? for they had made a great oath concerning him that came not up to the LORD to Mizpeh, saying, He shall surely be put to death .

i. e. A solemn Oath joyned with some terrible Execration against the Offenders herein.

Because by refusing to execute the Vengeance due to such Malefactors, they were justly presumed guilty of their Crime, and therefore liable to the same Punishment, as was the case of that City that would not deliver up an Idolater dwelling among them, to Justice.