In this chapter, all Israel unites together to destroy the men of Gibeah, but Benjamin rallies to defend their clansmen.
In general, I think the tribes of Israel are doing the right thing here. The phrase we saw over and over in Deuteronomy is "you shall purge the evil from your midst", and that's exactly what they decide to do. The entire town of Gibeah collaborated to commit this atrocity, so now the tribes of Israel align to destroy the men responsible. I don't think the Levite man did anything good, and in fact I'm pretty disgusted with him, but as he says in verse 5, the men of Gibeah would have killed him if he tried to stop them.
This chapter continues with the theme of "there was no king in Israel". The men of Benjamin rally to defend Gibeah, sticking to their tribal allegiances rather than justice and the Law. Benjamin is severely outnumbered, but win the battle on the first two days. The battle eventually goes in favor of the 11 tribes, in a way that reminds me of the battle against Ai in Joshua 8. In Joshua 7 and 8, Israel is defeated by the men of Ai after Achan sins against the LORD. The people kill Achan and then attack Ai, defeating it. Israel sets men in ambush, draw the men of Ai out of the city by feigning defeat, and then surrounding them and attacking on two sides.
This is exactly what happens here against Benjamin, who on the third day are drawn out of the town of Gibeah by the men of Israel feigning defeat, and then having an ambush destroy the city with fire. The men of Benjamin are terrified when they see that their retreat is cut off, and they attempt to flee but are nearly all slaughtered. What started as retribution against Gibeah ended in almost total destruction to the entire tribe of Benjamin, with only 600 survivors.
Unlike the battle of Ai, this results in the destruction of a tribe of Israel rather than an idolatrous nation of Canaan, so it's a Pyrrhic victory in that sense, but it's a battle that I feel they must have waged in order to fulfill the Law's demands for justice. Indeed, their war against Benjamin seems to have divine favor as they inquire of the LORD three times to ask whether they should attack Benjamin, and all three times the answer is yes.
Apart from the parallels to Ai, I think one of the interesting parts of this chapter is how Israel (i.e. all of the tribes except for Benjamin) is defeated by Benjamin twice even when they have the assent of the LORD. This is unusual because I don't think we saw Israel defeated by any of the Canaanites except for when Achan sinned and they were defeated by Ai. I've thought a lot about why the sons of Israel get defeated twice, and I can't really think of a theological explanation. We know that the Benjamites are skilled warriors, and they are renowned for their left-handed slingers both here and in other parts of the bible. Interestingly, Ehud from Judges 3 is also a left-handed Benjamite. A later example is 1 Chronicles 12:2 where the tribe of Benjamin fields ambidextrous slingers.
So even though Benjamin was outnumbered by 400,000 to 26,000, they win on the field through sheer prowess. In the end, the larger Israelite army has the LORD's support and they defeat Benjamin by laying an ambush and taking advantage of Benjamin's overconfidence. To us, I suppose this is a lesson about persistence. Even if the LORD is with us, and in favor of us doing something, it still might not work out at first. We have to avoid getting discouraged and continue to seek the LORD's guidance. After every time they are defeated, the men of Israel go back to the LORD weeping and asking if they are doing the right thing, attacking their brother Benjamin. Every time the answer is yes, and every time they go back out to fight against Benjamin again. They are doing a hard thing, attacking their fellow tribe, and that's why they continue to petition the LORD, because I'm sure they began doubting whether their path had the LORD's blessing. Perhaps they felt they were doing wrong, attacking their own brother, even though they knew that Benjamin had sinned by protecting Gibeah.
Israel began doubting, but they did the right thing and turned to inquire of the LORD every day whether they should attack Benjamin, and with his affirmative, they did so. So I believe that Israel did the right thing in this chapter, and Benjamin did the wrong thing by putting tribal affiliation and a desire to protect "their people" above justice and the Law of Moses. In the end, Benjamin is nearly destroyed because of it.