The ending chapters of Judges function at several levels:
1) historical context for the audience who received these stories in this form (the accounts refer to some time 1200-1100 BC). For instance, chapter 18 explains why Dan was in the north rather than in the south (where Joshua had said they were alloted land). Chapters 19-21 explain why Benjamin was so small and how they had barely survived. In the context of later generations reading this account it would explain the loss of tribes by means of the LORD expelling them for their continuing depravity. I am particularly thinking of the expulsion of the ten tribes of Israel (including Benjamin) in the 700s and then the later exile of Judah between 609-586 BC.
2) Kingship – the author of Judges is demonstrating what life without a king was like. The whole story (19-21) is framed by “there was no king” (19:1; 21:25). This would seem to indicate they had a positive appraisal of kingship even if the actual stories of kings for Israel and Judah does not play out that well (which might indicate that this account found its form in the days of David/Solomon).
3) Rejection of Benjamin – this whole story emphasizes the perversity of Benjamin and their near annihilation. We need to bear in mind that the king chosen first was from Benjamin. Is this a way of subtly (not so subtly) speaking against Benjamites ruling? After all, Saul would likely have been only a handful of generations removed from this incident. He owes his life to the sparing of the tribe, but also finds his genealogy littered with the perverse. More striking is that the father-in-law in Bethlehem of Judah (David’s hometown) is over-abounding in generosity toward the Levite (19:3-10). When the Levite finally leaves he is compelled by his servant to not stay in Jebus (what would become Jerusalem) because of the Jebusites whom they would not likely receive hospitality from. Instead they stay the night in Gibeah of Benjamin and are violated perversely.
4) Increasing depravity – the whole of judges undoes the work of Joshua. Joshua reads as if the people inherited the whole of the land. Judges (from the beginning) shows they did not. And not only was this because they did not deal with neighboring clans/tribes of the Canaanites as they should (and then face battles with various folks as judgment), but they even face assaults from their own tribes: Benjamin assaulting the concubine (and showing them to be just as evil as Sodom which was entirely destroyed) and the other tribes assaulting and almost completely destroying Benjamin. And the violence continues with the forcible taking of wives for Benjamin. And in the immediate account, the Levite treats the concubine with violence in his cutting her into pieces. And the text even is suggestive that the concubine hadn’t died from the gang raping and there is no clear indication she died prior to being cut up by the Levite. Is this demonstrating yet further that the Levites – those specifically responsible to teach and uphold Torah for everyone – were descended into depravity? (see Judges 18 about the Levite serving the idol stolen from Micah and established in Dan).
Arnold, Bill T., and H. G. M. Williamson, eds. Dictionary of the Old Testament: Historical Books. Downers Grove, IL: InterVarsity Press, 2005.
Block, Daniel Isaac. Judges, Ruth. NAC. Nashville, TN: Broadman & Holman Publishers, 1999.
Boling, Robert G. Judges. AB. New Haven, CT: Yale University Press, 2005.
Frolov, Serge. Judges. FOTL. Grand Rapids, MI: Eerdmans, 2013.
Hamilton, Victor P. Handbook on the Historical Books: Joshua, Judges, Ruth, Samuel, Kings, Chronicles, Ezra-Nehemiah, Esther. Grand Rapids, MI: Baker Academic, 2008.
Martin, Lee Roy. The Unheard Voice of God: A Pentecostal Hearing of the Book of Judges. Blandford Forum, Dorset, UK: Deo Pub, 2008.
Soggin, J. Alberto. Judges, a Commentary. OTL. Philadelphia, PA: Westminster Press, 1981.