What does Judges 21:3 mean?

Israel has slaughtered nearly the entire tribe of Benjamin. Only six hundred men of the tribe remain hiding in an area known as the "rock of Rimmon" (Judges 20:47–48). God directed Israel to fight against Benjamin (Judges 20:27–28) because of the depraved evils of the city of Gibeah (Judges 20:12–13). Nowhere did God command that Israel obliterate the entire tribe. And yet, the other eleven tribes appeal to God as if He is to blame for what has happened.

It's common for people to make mistakes, then appeal to God, asking "why did this happen?" or "why did you do this to me?" Here, the immediate answer for "why" a tragedy has happened is extremely direct: because that's what Israel chose to do. They clearly went well beyond what was necessary, or commanded, including making unwise promises (Judges 21:1). Yet now, they feel terrible about the looming extinction of one of the twelve tribes of their nation. They have spent the day following the slaughter weeping before the Lord at Bethel, where the ark of the covenant sits (Judges 20:26–27).

God does not answer their question, and the next day the people will convene again to work out a plan. That will be further complicated by a second oath which must be kept, as well (Judges 21:4–5).