The setup card for chapter 12 tells us that the first player should be (forgive the inexact quote - I don't own the game so am working off memory from 2 days ago):
the player who most wants to destroy Him
Our eventual interpretation from a quick group consensus was that it should be the player who had least often been the traitor (this narrowed it down to two of us, and we picked from there based on who had directly defeated the traitor in the previous scenario).
But some other possible ideas for it have occurred to me since, including the player with the most family deaths, the player who had least recently been the first player (purely for some fairness),
by consulting the helm, or the player with the most family crests on the helm.
Is there any established criteria that we should have used to determine starting player, or is what we did as correct as we could have gotten?