Matthew Poole and continuators on Judges 20:3

3 (Now the children of Benjamin heard that the children of Israel were gone up to Mizpeh.) Then said the children of Israel, Tell us , how was this wickedness?

Like Persons unconcerned and resolved, they neither went nor sent thither: partly from their own pride, and stubbornness, and self-confidence: partly because as they were loath to give up any of their Brethren to Justice, so they presumed the other Tribes would never proceed to a War against them: and partly, from a Divine infatuation hardening that wicked Tribe to their own Destruction.

The Verb is of the Plural Number, because they speak to the Levite, and his Servant, and his Host, who doubtless were present upon this occasion.