Quest. Why would God suffer them to have so great a loss in so good a cause? Answ. Because they had many and great sins reigning amongst themselves, and they should not have come to so great a work of God as this with polluted hands, but should have pulled the beam out of their own eye, before they attempted to take that out of their brother Benjamin's eye; which because they did not, God doth it for them, making them by this loss more clearly to see their own sins, and their need of God's help, without which their great numbers were insignificant; and bringing them through the fire, that they might be purged from their dross; it being probable that the great God, who governs every stroke in battles, did so order things, that their worst and rotten members should be cut off, which was a great blessing to the whole commonwealth.
Related Commentaries of Judges 20:21
Judges 20:21
21 And the children of Benjamin came forth out of Gibeah, and destroyed down to the ground of the Israelites that day twenty and two thousand men.