John Gill Commentary Judges 21:6

"And the children of Israel repented them for Benjamin their brother, and said, There is one tribe cut off from Israel this day." — Judges 21:6 (ASV)

And the children of Israel repented them for Benjamin their
brother
Not that they went to war with them, as if their cause was not good; but for the severity they had exercised towards them, especially in destroying their women and children, and for the fatal consequences like to follow hereafter, particularly the dissolution of the whole tribe:

and said, there is one tribe cut off from Israel this day ;
that is, there is a likelihood or great danger of it.