The people repented them for Benjamin, because that Yahweh had made a breach in the tribes of Israel.
"The action [against Jabesh-gilead] appears cruel in the extreme to the modern reader, but the virtual sacredness of the bond linking the several tribes into the amphictyony must be appreciated, and the sin of Jabesh-gilead seen in its light." [Note: Cundall and Morris, p. 210.] 