More than a war, the story reads like a case of internal strife and a case of revenge.
So a few men rape a concubine of a homeowner the whole night, who dies in the morning. When other tribes asked the Benjaminite tribe to give out those men to put them to death, they refused, and instead decided to war against them.
In the war, the Benjaminites end up getting killed and annihilated.
God’s arrangement was that each tribe had a group of Older men that were qualified to judge these kinds of cases.
Yes, it was God's arrangement. But what happens if the same judges shield the gang rapists? Remember, rather than punish the rapists, these men decided to wage war against the other Israelite tribes and even killed 22,000 of their men. On the second day, they again killed another 18,000 Israelites. What should the Israelites have done in reply?
What were these Benjaminite 'judges' doing in the city? The case was now not just judging those few rapists, but an entire tribe who was ignoring the existing laws.
Jehovah himself arranged for the justice and punishment to be meted out. The law is not bigger than the lawmaker.
Morally, there is no place in this world for people like Benjaminites who support and shield gang rapists.