The civil war that ends the book of Judges is harsh reading. It is one of the most jarring episodes in scripture. It shows the total degeneration of a culture. They are not being oppressed by an outside occupying foe. Israel is instead oppressed by their own abandonment of God. We notice a Danite priest with a huge following of idol worshippers. They assume this is somehow the worship of Yahweh. We find a Levite who chases down his own runaway concubine sex slave, showing the state of moral decline in the priestly class that was supposed to be leading the worship of the nation. We read about the lead city of Benjamin celebrating sinful sexuality of every kind in a large urban environment that is entertained by gang rape and murder. It is a tragic picture of a society characterized by abandonment of God and His Law.
Israel's "solution" to the decline as seen in Benjamin did not involve any return to God. Instead, it was vigilante justice, warfare, and the attempted genocide of an entire tribe of fellow Israelites. Benjamin was nearly eradicated by this civil war. Israel had become an absolutely chaotic culture. Everyone did what was right in his own eyes. Anarchy ruled.
The breakdown that unrepentant sin brings is costly, brutal, and gruesome to observe. Those caught in it have fallen so far, so gradually, that they may not even see the downfall. Only those who return to God and to understanding what He has said in His Word can be spared the further damage of the decline.