The man that went to retrieve his concubine and then saved himself by giving her over to be gang raped by the Benjaminites.
Then he was miffed that she died because of it. Such a loss of property! Then he cut her up and sent the pieces to the other tribes to show how much he was upset over it.
They showed their righteous indignation over it and killed most the Benjaminites. Oh yeah, their domestic animals too. Women don't get mentioned but they must have been killed too because the few remaining Benjaminites, didn't have any women to marry.
So because one group didn't swear not to give any of their daughters to the benjaminites, their unfortunate daughters were kidnapped by the leftover benjaminites who didn't mind gang raping women.