Adam Clarke on Judges 20:13

Deliver us the men - Nothing could be fairer than this. They wish only to make the murderers answerable for their guilt.

Benjamin would not hearken - Thus making their whole tribe partakers of the guilt of the men of Gibeah. By not delivering up those bad men, they in effect said: "We will stand by them in what they have done, and would have acted the same part had we been present." This proves that the whole tribe was excessively depraved.