John Gill Commentary Judges 20:9

"But now this is the thing which we will do to Gibeah: [we will go up] against it by lot;" — Judges 20:9 (ASV)

But now this shall be the thing which we will do to Gibeah ,
&c.] Where the fact was done; what follows was proposed by some, and unanimously agreed to by all:

we will go up by lot against it ;
cast lots who shall go up to it and demand satisfaction for the offence committed; and if denied, to act in an hostile manner against it.