They just did this to make sure everyone possible was included. If they'd filmed all the usual suspects and Stacey it might have been obvious that the outcome would be Stacey.
I can think of a motive for Max (though it's pretty weak) - say Bradley had rushed off Christmas Day and told Max about the rape after Stacey had told him. He may even have confessed hitting Archie and told him what Archie'd said - I can't remember what it was exactly but it was very mean. Then Max may have gone to confront Archie and killed him, because he cares for Stacey as well as Bradley.
Just a thought, anyway![]()